U.S. sanctions Turkey for purchasing Russian defense system in 2017

The big picture: Even as Turkey has taken on an increasingly belligerent foreign policy, angering other countries in NATO, Erdoğan has largely been shielded from U.S. punitive action thanks to his close personal ties with President Trump.

• In spite of Trump's reservations, the U.S. removed Turkey from its F-35 stealth fighter jet program last year after it began receiving Russia's S-400 system, which Washington fears could be used to gather intelligence on the F-35 and its weaknesses.
• The Trump administration had previously held off implementing sanctions against Turkey for violating the 2017 Countering America’s Adversaries through Sanctions Act, which requires the U.S. to sanction countries trading with Russia's defense and intelligence sectors.

Worth noting: The 2020 National Defense Authorization Act, which Trump has threatened to veto for unrelated reasons, would have forced the administration to impose sanctions on Turkey within 30 days.

What they're saying: “The United States made clear to Turkey at the highest levels and on numerous occasions that its purchase of the S-400 system would endanger the security of U.S. military technology and personnel and provide substantial funds to Russia’s defense sector, as well as Russian access to the Turkish armed forces and defense industry,” Secretary of State Mike Pompeo said in a statement.

The other side: Turkey's foreign ministry condemned the decision and said it would "not refrain from taking measures that it deems to be necessary to ensure national security."

Go deeper: Past friction between Biden and Erdoğan foreshadows future tensions