Last week, the United States removed Turkey from the F-35 fighter jet program after Russia began delivering its S-400 missile defense system to Turkey. This move is significant since Turkey produces parts for the jet and was due to receive at least 22 of the U.S. jets by 2022.

The weapons deal with Russia, valued at $2.5 billion, has been in the works since 2017. A White House spokesperson said that “accepting the S-400 undermines the commitments all NATO allies made to each other to move away from Russian systems.”

The disagreement over arms sales belies a deeper political conflict between Turkey — a longtime NATO member — and the broader NATO alliance. Here are three takeaways for ongoing U.S., Turkey and NATO relations:

1. The S-400 isn’t compatible with NATO’s arsenal and could be used to collect sensitive data

U.S. and NATO defense officials have said the S-400 is incompatible with NATO’s systems in the region — but also are concerned about potential data and security breaches that might allow the Russian military to access NATO operational secrets. In a statement last week, the White House said, “The F-35 cannot coexist with a Russian intelligence collection platform that will be used to learn about its advanced capabilities.”

NATO’s perspective is that Turkey can have the S-400 or the F-35, not both. German officials say the deal raises “difficult questions” for the alliance. U.S. defense secretary nominee Mark Esper called Turkey’s decision on the S-400 “the wrong one.” Israel, though not a NATO member, questioned whether Turkey could still be considered a “real” member of the alliance.

The U.S. move means Turkish pilots and technicians training on the F-35 in the United States will be sent home by the end of the month. If Turkey ever does receive the F-35, its military will lag behind other NATO allies in training and readiness. This also will negatively affect Turkey’s ability to conduct joint operations with NATO.

2. The S-400 purchase could foreshadow Turkey’s break with NATO

This isn’t the first time Turkey has used a weapons purchase to exert its independence from NATO. In 2013, Turkey signed a preliminary deal with China for the FT-2000, a missile defense system that had never been exported, and whose capabilities were largely untested. Turkey put the deal on hold in 2015, but a Turkish official cautioned NATO allies, “Our procurement decisions are not free of deliberations on foreign policy.” In this case, Turkey was watching how the United States and France would handle events surrounding the 100-year anniversary of the Armenian genocide, which Turkey does not recognize.

Turkey joined NATO in 1952, in part because of shared security concerns about the Soviet Union. In a bipartisan statement last week, the leadership of the Senate Armed Services and Foreign Relations Committees said “by accepting delivery of the S-400 …[President Recep Tayyip Erdogan] has chosen a perilous partnership with Putin at the expense of Turkey’s security, economic prosperity and the integrity of the NATO alliance.” To these U.S. senators, the deal is evidence that Erdogan “remains fixated on deepening ties with Vladimir Putin.” Earlier this year, Vice President Pence said that the United States “will not stand idly by while NATO allies purchase weapons from our adversaries,” potentially setting up a situation in which Turkey must choose between its membership in NATO and its weapons from Russia.

No NATO members have left since the alliance’s founding in 1949, and a number of new countries have joined in recent decades. So what does Turkey’s S-400 purchase mean to NATO members? As one scholar remarked, “It’s a political statement that the U.S. doesn’t have hegemony and control over NATO members and particularly over Turkey.”

3. Dropping Turkey from the F-35 program could push Turkey closer to Russia

Turkey welcomed the F-35 with pomp and circumstance in June 2018; the rollout ceremony highlighted Turkey’s links to the NATO allies who helped produce the jet. The news that Turkey is no longer part of the F-35 program means the Turkish military will have to search for an alternate stealth fighter. Russia has already offered Turkey the Su-35, and Turkey might become the first export market for Russia’s advanced — though troubled — Su-57 stealth fighter.

Turkey’s Foreign Ministry cautioned that dropping Turkey from the F-35 program would “cause irreparable wounds in strategic relations” with NATO. If Turkey does receive advanced jets from Russia, it is difficult to imagine Turkey and NATO repairing this rift, especially if Russia continues to stand ready with additional weapons offers.

Although cutting Turkey out of the F-35 program was intended to show the consequences of buying Russian weapons, the U.S. move might end up accelerating Turkey’s alignment with Russia. This would be in line with Erdogan’s goals, including, as he outlined in a New York Times editorial last summer, forcing the United States to “give up the misguided notion that our relationship can be asymmetrical and come to terms with the fact that Turkey has alternatives.”

The S-400 purchase is Erdogan’s clear demonstration that Turkey has alternatives to U.S. and NATO weapons, and perhaps to the U.S. and NATO altogether. Erdogan is also pursuing joint production of the S-400, a move that would bring Russian and Turkish interests into closer alignment.

But this realignment could leave NATO at risk of losing an important strategic and military ally. NATO forces are active in two locations in Turkey related to the ongoing crisis in Syria, a conflict in which NATO and Russia have competing interests. NATO and Russia both have strategic interests in Turkey, and air bases in Turkey have been an important staging ground for NATO and U.S. operations in Iraq and Afghanistan.

A closer relationship between Turkey and Russia risks NATO and U.S. access to these bases, as well as land and sea routes. Coming on the heels of Russia’s annexation of Crimea, NATO has reasons to be concerned about losing Turkey to Russia’s influence.

Jennifer Spindel is an assistant professor of international security at the University of Oklahoma, and the associate director of the Cyber Governance and Policy Center. Follow her on Twitter @jsspindel.