For his part, Mr. Erdogan has celebrated the S-400 purchase — and, to a lesser degree, his own defiance — as a sign of Turkey’s increasing independence on the world stage.

“As long as we, as a nation, protect our homeland, our flag, the call for prayer, democracy and the state,” Mr. Erdogan said on Monday during the anniversary celebration of a failed coup attempt against him in 2016, “God willing, no power’s hand will be able to reach them.”

Mr. Trump has in the past praised Mr. Erdogan as an ally in the fight against terrorism while ignoring the Turkish leader’s authoritarian crackdown on his own people. When Mr. Erdogan visited the White House in 2017, the two hailed the arrival of a new era of relations after the presidency of Barack Obama.

This week, Mr. Erdogan appeared ready to trade on that relationship by suggesting that Mr. Trump had the power to waive any sanctions that would arise over the purchase.

Asked on Tuesday whether he would indeed impose sanctions, Mr. Trump deflected by blaming the Obama administration for failing to sell the Patriot missile systems to Turkey. The Obama administration did, in fact, consider selling Patriot missiles to Turkey, but negotiations were repeatedly scuttled.

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The White House statement on Wednesday said the Trump administration “made multiple offers to move Turkey to the front of the line.” But a deal never came together.

At the Pentagon on Wednesday, officials referred questions about potential American sanctions against Turkey to the State Department, which did not immediately respond to a request for comment.