The United States is finally punishing Turkey for purchasing a Russian missile defense system, a long-anticipated move that is likely to increase tensions with a NATO ally.

On Monday, the Trump administration imposed sanctions on Turkey for its purchase of the Russian-made S-400 surface-to-air missile defense system. The administration ordered the penalties under a section of the Countering America’s Adversaries Through Sanctions Act (CAATSA), which gives the president the power to sanction people or entities that do business with Russia’s intelligence or defense sectors. The sanctions specifically target Turkey’s defense procurement agency, known as the Presidency of Defense Industries (SSB), and its senior officials.

“Despite our warnings, Turkey moved ahead with its purchase and testing of the S-400 system from Russia,” Secretary of State Mike Pompeo said in a statement Monday. “Today’s sanctions on Turkey’s SSB demonstrates the U.S. will fully implement #CAATSA. We will not tolerate significant transactions with Russia’s defense sector.”

But these sanctions are kind of overdue. Turkey acquired the defense system last year, after repeated warnings by the Trump administration not to do so because they do not want a NATO ally relying on Russian systems. US officials also said Turkey’s use of the S-400 jeopardized America’s F-35 fighter jet program, over fears the Russian system’s radars could collect intelligence on the F-35s.

In response, the US removed Turkey from its F-35 fighter jet program, which barred the country from getting the jets and restricted any Turkish personnel from working with the planes. Still, bipartisan members of Congress continued to push for harsher punishment of Turkey, including sanctions.

Turkey’s decision to purchase the Russian system further strained relations between Washington and Ankara. Turkish President Recep Tayyip Erdoğan has consolidated power in recent years, becoming more explicitly authoritarian and cracking down on dissent, including by jailing journalists and others he perceives as his political enemies.

Erdoğan was angered by the US’s decision to ally with the Kurds in the fight against ISIS in Syria, as Erdoğan associates them with the Kurdistan Workers’ Party, a terrorist group that’s waged attacks in Turkey. He has also bristled at the US’s refusal to extradite a US-based cleric whom Erdoğan blames for a 2016 coup attempt.

Beyond Turkey’s flirtation with Russia, Turkey has also tried to exert its regional influence in places like Syria and Libya and the eastern Mediterranean Sea, where its gas exploration efforts have increased tensions with Greece and other NATO allies in the European Union, too. (The EU is also considering sanctions against Turkey.)

But despite issuing lots of admonitions, the Trump administration didn’t move forward with the CAATSA sanctions. Some attributed Trump’s refusal to do so to his personal affinity for Erdoğan.

Then in October, Turkey tested the S-400 system in defiance of US warnings, making it much harder for the US to ignore.

And this month, Congress, in its annual defense authorization bill, included mandatory sanctions against Turkey for its Russian defense shopping spree. Though Trump has threatened to veto the bill for lots of reasons, the administration’s move to sanction Turkey on Monday may have been an attempt to get ahead of that requirement.

Either way, the imposition of sanctions on Turkey is only going to deepen the divide between the two NATO allies. Turkey’s economy was struggling badly before the coronavirus, and the financial penalties could add to that pain.

Erdoğan condemned the sanctions. “We expect support from our NATO ally, the United States, in our fight against terrorist organizations … not sanctions,” he said, according to CNN Türk.

But the Trump administration’s decision today is really going to be President-elect Joe Biden’s problem. Biden has previously called Erdoğan an “autocrat” and expressed concern about Turkey’s growing closeness with Russia. And in October, Biden called on the Trump administration to pressure Turkey over its provocations in the eastern Mediterranean.

Biden and other allies face a difficult dilemma. Turkey is a strategic NATO partner, and the US and EU want to put pressure on Erdoğan to try to stop his misbehavior and bring him back into the fold. But too much pressure may backfire, pushing Ankara and Moscow even closer together.

As Biden seeks to rebuild American partnerships, the relationship with Turkey may be one of the trickiest.