Holding the Southern Flank: Recalibrating the United States-Türkiye Alliance Published May 14, 2026 By Lt. Col. Erin M. Yancey, AWC The North Atlantic Treaty Organization (NATO) continues to be the cornerstone of American strategy in Europe and the Mediterranean, providing the institutional framework through which the United States projects collective defense, deters aggression, and maintains the political unity of the transatlantic community.[1] Recent United States strategic guidance acknowledges that alliances are essential to advancing American interests and that sustaining deterrence increasingly depends on integrating allied military capabilities, coordinating burden-sharing, and aligning diplomatic and economic instruments of power.[2] In an era defined by renewed great-power competition, alliance strength relies less on any individual capability than on the alignment of its members’ political will, threat perceptions, and military cooperation. When that alignment falters, the alliance's credibility and its deterrent power start to decline. On NATO’s southern flank, this decline is now visible. One of the alliance’s most strategically important members, Türkiye, is increasingly diverging from the United States in both political views and military practices. The United States-Türkiye security relationship, once a key pillar of NATO deterrence in the Eastern Mediterranean, the Black Sea, and the Middle East, has deteriorated into a transactional partnership marked by mistrust, operational disagreements, and strategic differences. This fracture is not a temporary diplomatic dispute. Rather, it reflects a structural political–military conflict between Ankara and Washington. Türkiye increasingly views Kurdish autonomy in northern Syria as an existential threat to the Turkish state, while the United States has relied on Kurdish-led forces as its most effective partner against the Islamic State (ISIS).[3] These competing imperatives have created an alliance security dilemma in which actions taken by one ally to enhance its security are perceived by the other as a direct threat. This divergence, rooted in the interaction between Türkiye’s domestic political imperatives and the operational realities of the Syrian theater, undermines NATO cohesion, jeopardizes counterterrorism gains in Syria, and creates openings for strategic competitors to expand influence along the alliance’s southern flank. If left unmanaged, these dynamics risk transforming a strained alliance into a persistent strategic liability. Addressed to the Commander of United States European Command (USEUCOM), this white paper argues that the United States must pursue a deliberate strategy to reintegrate Türkiye into the Western security architecture while managing areas of unavoidable divergence. It recommends a synchronized military and interagency strategy focused on resolving the S-400 interoperability crisis, establishing a coordinated framework to manage the Syrian border dilemma, and expanding defense cooperation that reinforces Türkiye’s role in NATO deterrence. These military initiatives must be supported by coordinated economic, diplomatic, and allied actions that address the political and structural drivers of Turkish strategic hedging. Understanding how this divergence emerged is essential to determining how to mitigate it. The deterioration of the United States-Türkiye partnership is the product of broader structural changes in Türkiye’s domestic politics and regional security environment. Over the past decade, Türkiye’s internal political trajectory has fundamentally reshaped its foreign and security policy behavior. Under President Recep Tayyip Erdoğan, the Justice and Development Party (AKP) has increasingly relied on nationalist coalition politics to maintain power, a shift that has transformed domestic political survival into a central driver of foreign policy decision-making.[4] Within this political environment, Kurdish political movements have been reframed as existential threats to Turkish sovereignty. This perception is reinforced by a long-standing historical narrative known as the “Sèvres Syndrome,” which reflects a deeply rooted belief that external powers seek to fragment the Turkish state by empowering minority populations.[5] In contemporary Turkish politics, this narrative has regained prominence as democratic backsliding and political polarization have narrowed the regime’s governing coalition.[6] Kurdish autonomy, therefore, is not interpreted by Türkiye as a localized security issue but as a precedent that could reignite separatist movements within its borders. This political dynamic conflicts directly with American operational strategy in Syria. After ISIS expanded its territory in 2014, the United States aimed to dismantle the organization without deploying large numbers of ground forces. The Syrian Democratic Forces (SDF), led primarily by Kurdish People’s Defense Units (YPG), served as a highly capable partner for this effort.[7] With United States airpower, intelligence support, and advisory assistance, the SDF played an instrumental role in dismantling ISIS’s territorial caliphate and securing critical detention facilities housing thousands of ISIS fighters.[8] From Ankara’s perspective, however, the partnership was strategically intolerable. Turkish officials see the YPG as inherently linked to the Kurdistan Workers’ Party (PKK), a group that has waged a decades-long insurgency against the Turkish state and is labeled a terrorist organization by both the United States and the European Union.[9] Therefore, American support to Kurdish forces in Syria is viewed in Türkiye not as a tactical counterterrorism measure but as indirect support for a hostile militant network. Actions taken by the United States to defeat ISIS strengthened a Kurdish force that Türkiye viewed as a direct security threat. In response, Türkiye launched a series of cross-border military operations, including Euphrates Shield, Olive Branch, and Peace Spring, aimed at dismantling Kurdish autonomy along its southern border.[10] By establishing “safe zones” and resettling Syrian refugees, Turkish leaders sought to simultaneously counter Kurdish influence and address mounting domestic dissatisfaction over the refugee crisis.[11] As these dynamics intensified, military cooperation between the United States and Türkiye increasingly became entangled with domestic political narratives. Military operations in Syria were not merely tactical actions but powerful symbols reinforcing narratives of sovereignty and resistance to Western interference.[12] At the same time, defense procurement decisions evolved into political signals of strategic independence. The most consequential example was Türkiye’s acquisition of the Russian S-400 air defense system in 2019.[13] While Türkiye justified the purchase as a necessary defensive capability, the decision carried profound strategic implications for NATO interoperability.[14] The United States responded by removing Türkiye from the F-35 program and imposing sanctions under the Countering America’s Adversaries Through Sanctions Act (CAATSA).[15] The result was not simply a bilateral disagreement but a degradation of NATO’s integrated air defense architecture. Integrated deterrence depends on shared networks, interoperable platforms, and coordinated military planning. The S-400 dispute has become the most visible manifestation of the alliance’s broader strategic fracture.[16] Beyond the immediate military implications, the decline in the American-Turkish relationship also carries broader consequences for the United States’ national security strategy. Türkiye holds a strategically vital geographic position at the intersection of Europe, the Middle East, and the Black Sea. Its territory contains essential NATO infrastructure, such as early-warning radar systems and important logistical hubs that support regional operations.[17] Ongoing deterioration in alliance relations could weaken the United States’ ability to project power and maintain deterrence across multiple regions. At the same time, strategic competitors are increasingly exploiting the growing divide. Türkiye maintains deep economic and energy ties with Russia. They have declined to fully participate in Western sanctions against Moscow following the invasion of Ukraine.[18] Simultaneously, Ankara has expanded economic engagement with China and explored participation in alternative international institutions such as the Shanghai Cooperation Organization and BRICS-related economic frameworks.[19] Although Türkiye remains formally committed to NATO, these parallel relationships illustrate an emerging hedging strategy that complicates alliance cohesion. Despite these challenges, opportunities still exist to reintegrate and strategically realign the alliance relationship.. Recent regional developments suggest that the strategic divergence between Washington and Ankara may not be irreversible. The evolving political landscape in Syria following the collapse of the Assad regime has reshaped the security environment along Türkiye’s southern border, creating both new risks and opportunities for alignment between NATO allies.[20] At the same time, the PKK’s announced dissolution and subsequent disarmament have created the first credible off-ramp in decades for Ankara to reduce threat inflation without appearing weak.[21] Political signals from Kurdish actors regarding the future of the PKK insurgency and the evolving status of Kurdish-led forces in Syria suggest that long-standing security grievances between Türkiye and the United States may be entering a period of transition.[22] These developments create a narrow but significant window for the United States to recalibrate its approach to Türkiye before structural divergence hardens into long-term strategic separation. These openings will not last if the United States treats them as background noise. Recent events have highlighted how quickly contingency can become crisis on NATO territory. In March 2026, Türkiye reported a ballistic missile launched from Iran was intercepted by NATO air defenses, debris falling near Hatay, uncomfortably close to the strategic gravity of Incirlik and broader allied posture.[23] Whether or not every detail becomes durable history, the strategic lesson is immediate: the southern flank is a live theater where escalation, basing politics, and alliance credibility can collide in days, not months. The operational requirement is to seize this moment to execute a coherent, conditions-based campaign to restore alliance functionality, one that links objectives and effects across organizations, aligns higher strategy, and remains adaptive through assessment.[24] USEUCOM plays a central role in this effort as the combatant command responsible for the military relationship with Türkiye and for the defense of NATO’s southeastern flank. Consistent with the 2026 National Defense Strategy (NDS), effective deterrence increasingly depends on integrating allied capabilities, coordinating burden-sharing, and operating with partners across multiple domains.[25] Failure to maintain cohesion among key allies weakens deterrence by creating seams that adversaries can exploit. For this reason, restoring functional military cooperation with Türkiye is not simply a bilateral issue but a strategic requirement for maintaining NATO’s credibility in an era of great-power competition. This means acting now on four lines of effort. First, USEUCOM should lead a verifiable technical resolution pathway for the S-400 crisis that restores NATO interoperability and removes the primary structural obstacle to defense cooperation. The goal is not rhetorical progress but measurable results; USEUCOM should establish a bilateral military-technical working group with the Turkish General Staff to identify a supportable plan that renders the S-400 system permanently inoperable. Possible options include dismantlement or relocation to a controlled testing facility outside operational networks.[26] Turkish leadership is openly signaling that lifting CAATSA is a priority, and the issue remains tied to the broader American-Turkish defense-industrial future.[27] Resolving the S-400 dispute would allow the United States to lift CAATSA sanctions and restore defense industrial cooperation, including the modernization of Türkiye’s aging F-16 fleet.[28] In recent months, the urgency of this issue has grown as Turkish officials have pursued renewed negotiations with the United States over sanctions relief and the purchase of fighter aircraft.[29] This underscores Türkiye’s persistent interest in preserving its ties to Western defense systems rather than fully shifting to Russian or Chinese options. While addressing the S-400 dispute is necessary, it is not sufficient. The second, deeper challenge lies in the operational disconnect between American military activities in Syria and Türkiye’s security concerns. Currently, the United States manages these relationships through separate combatant commands, with USEUCOM responsible for alliance relations with Türkiye and United States Central Command (USCENTCOM) responsible for counter-ISIS operations alongside Kurdish-led forces in Syria.[30] This division has historically produced fragmented policy implementation and inconsistent signaling to regional partners. Joint Publication (JP) 1, Volume 1, underscores that combatant command campaigns and contingencies must be integrated rather than executed in isolation.[31] To address this gap, USEUCOM should advocate for establishing a standing EUCOM–CENTCOM coordination task force to manage the evolving security environment in northern Syria. This joint task force would coordinate intelligence sharing and establish verification mechanisms to prevent cross-border escalation. JP 5-0 notes that an effective strategy requires synchronizing military operations with diplomatic and interagency efforts to achieve unity of effort across theaters.[32] A formalized EUCOM–CENTCOM coordination framework would allow the United States to maintain counterterrorism pressure on ISIS while simultaneously addressing Türkiye’s security concerns regarding Kurdish militant groups. Third, at the same time, USEUCOM should expand security cooperation initiatives that redirect alliance attention toward shared strategic threats. Reinforcing common external challenges remains one of the most effective ways to stabilize strained alliances.[33] Recent regional events underscore the importance of such cooperation. Missile and drone threats originating from regional conflicts have increasingly threatened NATO territory and maritime infrastructure in the Eastern Mediterranean.[34] In this context, Russian military activity in the Black Sea and Eastern Mediterranean provides a natural focal point for renewed cooperation.[35] Expanding joint exercises focused on maritime domain awareness, anti-submarine warfare, and integrated air and missile defense would highlight Türkiye’s indispensable role in NATO deterrence. Enhanced air and missile defense integration between Türkiye and NATO allies would significantly strengthen the alliance’s ability to respond to these emerging threats. By elevating Türkiye’s operational role in NATO’s southeastern defense architecture, USEUCOM can reinforce Ankara’s strategic value to the alliance while encouraging greater alignment with Western security priorities. Even as the United States pursues reintegration, a prudent strategy requires maintaining operational flexibility should the alliance relationship continue to deteriorate. The fourth line of effort should focus on further developing alternative basing infrastructure with regional partners, such as Greece, while preserving the operational utility of current Turkish facilities.[36] Maintaining redundancy ensures that American military operations remain resilient regardless of future political developments in Türkiye. This approach aligns with joint doctrine that emphasizes the importance of an adaptable force posture and distributed basing in contested strategic environments.[37] In practice, USEUCOM should continue infrastructure hardening where access is secure, expand dispersal options, and ensure sustainment pathways do not hinge on a single political decision point. Military initiatives alone, however, cannot resolve the deeper political and economic drivers of Turkish strategic behavior. Stabilizing the alliance relationship requires a coordinated approach that integrates military engagement with diplomatic initiatives, economic incentives, and allied cooperation. The political dynamics driving Türkiye’s regional policies, particularly regarding Syria, cannot be mitigated through defense cooperation alone. Instead, the United States must align military engagement with political efforts. Diplomatic engagement on Syria’s future represents a decisive line of effort for interagency coordination. The collapse of the Assad regime and the emergence of transitional authorities have created a fluid political environment in which external actors retain significant influence over governance outcomes.[38] Within this environment, Türkiye is not a peripheral actor; it is a central power broker. Through years of political support, humanitarian assistance, and military involvement, Türkiye has developed extensive relationships with Syrian opposition groups and holds considerable leverage over emerging transitional institutions. This influence provides both an opportunity and a risk for American strategy. Türkiye holds a key position as a stakeholder capable of shaping post-conflict governance arrangements. However, if left unmanaged, Turkish priorities, particularly regarding Kurdish political representation, could produce governance outcomes that exacerbate instability. The United States should engage Türkiye as a central partner in shaping political arrangements that prevent extremist safe havens while preserving regional stability. Coordinated diplomacy can ensure that stabilization efforts in Syria reinforce, rather than undermine, alliance security interests. The deterioration of the United States-Türkiye relationship represents one of the most consequential strategic challenges facing NATO’s southern flank. Driven by the interaction of domestic political imperatives in Ankara and operational realities in Syria, the alliance has entered a cycle of mistrust that undermines interoperability, weakens deterrence, and creates opportunities for strategic competitors. Yet the alliance has not collapsed. Türkiye remains a NATO member with immense geographic, military, and political importance, controlling access to the Black Sea, supporting the alliance’s southeastern defense system, and serving as a key link between Europe and the Middle East. The strategic challenge for the United States is therefore not to force alignment where interests diverge, but to rebuild cooperation where strategic interests still converge. By resolving the S-400 crisis, coordinating military strategy across combatant commands, expanding joint deterrence initiatives in the Eastern Mediterranean, and leveraging interagency and allied cooperation, the United States can stabilize the alliance relationship while addressing the underlying drivers of Turkish strategic hedging. These steps will not eliminate every source of friction between Washington and Ankara, but they can restore the functional military cooperation necessary for NATO’s collective defense posture. The stakes extend far beyond the bilateral relationship. In an era defined by renewed competition with Russia and China, alliances remain the United States’ greatest asymmetric advantage. A fractured southern flank would weaken NATO’s credibility, complicate regional deterrence, and provide strategic competitors with opportunities to exploit divisions within the alliance. Repairing the relationship with Türkiye will require patience, realism, and sustained engagement. But failure to act risks allowing a structural fracture within NATO to harden into a permanent strategic vulnerability. The question before American policymakers is no longer whether the United States–Türkiye alliance can return to the past. The real question is whether the United States will act decisively enough to keep one of NATO’s most strategically positioned allies anchored within the alliance, because if the southern flank fractures, NATO will not be weakened first by Russia or China, but by its own failure to hold the line. Acknowledgments: I would like to thank Maj Scott Yancey for his review and editing of the paper. Lt. Col. Erin Yancey is a dentist and officer in the U.S. Air Force with a career spanning over 16 years. After receiving her commission in 2010, she has become a board-certified General Dentist, earned a Master of Science in Oral Biology, and has held numerous leadership positions, including Chief of Dental Services. A published researcher with papers on restorative composites and light curing, she is currently a student at the Air War College, pursuing a Master of Strategic Studies. Throughout her career, she has remained dedicated to advancing the oral health of Airmen and their families, mentoring junior officers, and ensuring operational readiness through exceptional dental care. [1] Jim Zanotti and Thomas Clayton, Turkey (Türkiye): Major Issues and U.S. Relations (Washington, DC: Congressional Research Service, 2025), Introduction. [2] Donald J. Trump, National Security Strategy of the United States of America, Washington, DC: The White House, 2025. [3] Zanotti and Clayton, Turkey (Türkiye). [4] Ibid. [5] Şener Aktürk, Regimes of Ethnicity and Nationhood in Germany, Russia, and Turkey, Cambridge: Cambridge University Press, 2012, p 169-175. [6] Maurus Reinkowski, Chapter 4 The Promise of Islamic Conservatism, 1980-2013, The History of Turkey: Grandeur and Grievance, 2023. [7] Zanotti and Clayton, Turkey (Türkiye). [8] Charles Lister, The Syrian Jihad: Al-Qaeda, the Islamic State and the Evolution of an Insurgency, Oxford: Oxford University Press, 2016. [9] Gareth Jenkins, The PKK: A Historical and Political Analysis, London: Routledge, 2017. [10] Aaron Stein, “Turkey’s S-400 Gamble,” Survival 61, no. 4 (2019), p 7–22. [11] Reinkowski, The History of Turkey, p 280-284. [12] Zanotti and Clayton, Turkey (Türkiye). [13] Ibid. [14] F. Stephen Larrabee and Ian Lesser, Turkish Foreign Policy in an Age of Uncertainty, Santa Monica, CA: RAND Corporation, 2021. [15] Zanotti and Clayton, Turkey (Türkiye). [16] U.S. Department of Defense, 2022 National Defense Strategy of the United States, Washington, DC: Department of Defense, 2022. [17] Zanotti and Clayton, Turkey (Türkiye). [18] Ibid. [19] Aslı Aydıntaşbaş and Jeremy Shapiro, “The U.S.–Turkey Relationship After Erdoğan,” European Council on Foreign Relations, 2023. [20] UK House of Commons Library, “Syria after Assad: Consequences and interim authorities,” July 23, 2025. [21] Associated Press, “Imprisoned PKK leader calls for new laws that would advance peace…,” February 2026. [22] Zanotti and Clayton, Turkey (Türkiye). [23] Tuvan Gumrukcu,“Turkey says NATO defences destroyed Iran missile, risking war expansion,” Reuters, March 4, 2026. [24] Joint Chiefs of Staff, JP 5-0, Joint Planning [25] U.S. Department of Defense, 2026 National Defense Strategy, (Washington, DC: Department of Defense, 2026). [26] Zanotti and Clayton, Turkey (Türkiye). [27] Reuters, “Turkey eyes lifting U.S. CAATSA sanctions before U.S. midterms,” March 3, 2026. [28] Zanotti and Clayton, Turkey (Türkiye). [29] Ibid. [30] Ibid. [31] Joint Chiefs of Staff, JP 1, Vol. 1, Joint Warfighting, “Global integration” and campaigning integration. [32] Joint Chiefs of Staff, JP 5-0, Joint Planning [33] Stephen M. Walt, The Origins of Alliances, Ithaca, NY: Cornell University Press, 1987, p 17–26. [34] Ian Williams et al., Missile Threat: Regional Missile Proliferation and Security Implications, Washington, DC: Center for Strategic and International Studies, 2023. [35] Dmitry Gorenburg, Russia’s Interests in the Black Sea and Mediterranean, Arlington, VA: CNA Corporation, 2025. [36] Zanotti and Clayton, Turkey (Türkiye), p 12-13. [37] Joint Chiefs of Staff, JP 1, Vol. 1, Joint Warfighting. [38] UK House of Commons Library, Syria after Assad.