U.S.–Turkey ties slide further amid advanced weapons impasse

The U.S. removed Turkey from the F-35 fighter jet program on Wednesday, escalating a months-long standoff over Turkey's purchase of Russian S-400 air defense systems.

Why it matters: Turkey is a strategic U.S. ally in the Eastern Mediterranean and Central Asia and an important partner in American relations with the Muslim world. President Recep Tayyip Erdogan’s decision to move forward with the S-400 purchase risks undermining NATO military coordination and exposing U.S. and broader NATO alliance capabilities to Russian intelligence.

Background: Turkey has seen several recent U.S. decisions as running counter to its security interests.

• During the fight against ISIS, the Obama administration oversaw the ascendence in northeast Syria of the YPG — an iteration of the Kurdistan Workers' Party (PKK), which Turkey views as an existential threat.
• After the failed coup in July 2016, Turkey sought a more forceful response by Washington, including the potential extradition of exiled cleric Fethullah Gulen, the figurehead of the cult alleged to be behind the coup attempt.
• Just last week, bipartisan leaders of the Senate Armed Services and Foreign Relations committees issued a statement supporting sanctions against Turkey for its drift toward Russia.

Between the lines: As Ankara has lost priority in the U.S.' Middle East strategy, Erdogan has sought to reconfigure Turkey's sphere of influence, including warmer diplomatic and economic relations with Moscow — a move some of the U.S.' Arab allies have made as well.

• For Turkey, the S-400s are a slight at the U.S., a hedge against NATO security guarantees Erdogan considers weakened, and a symbol of Russia's success in outcompeting U.S influence.
• Putin has been able to use the situation in Idlib, Syria, as leverage over Erdogan by holding back forces loyal to President Bashar al-Assad from retaking the city. Turkey has taken in nearly 4 million Syrian refugees and does not want to see the humanitarian crisis on its southern border exacerbated.
• Erdogan, meanwhile, is capitalizing on Turkey's role as the geographic buffer between Europe and Syrian refugees to threaten the U.S., should relations deteriorate further.

What to watch: Many of Turkey's grievances, including legitimate strategic neglect of Ankara by Washington, pre-date Trump's presidency. But the next chapter of the U.S.–Turkey alliance will be shaped by the unpredictable dynamics of strongman diplomacy between Trump and Erdogan.

Adham Sahloul is a foreign policy analyst and former researcher at the Atlantic Council's Rafik Hariri Center for the Middle East.