What did the first CAATSA sanctions on an ally mean?

What did it mean when Washington turned a sanctions law written against Russia on a fellow ally? On 15 December 2020, a day after the announcement, that question was the one left hanging over the decision on Turkey.

On 14 December 2020, Secretary of State Mike Pompeo announced that the United States had imposed sanctions on Turkey over its acquisition of the S-400 missile defence system from Russia. As reported at the time, it was the first occasion on which the Countering America's Adversaries Through Sanctions Act, known as CAATSA, had been used to penalise a US ally.

The law behind the move

CAATSA is a federal law that imposed sanctions on Iran, North Korea and Russia. It works by preventing US companies from doing business with sanctioned entities. The Senate passed it on 27 July 2017 by 98 votes to 2, after the House approved it by 419 to 3. President Donald Trump signed it on 2 August 2017, though he believed the legislation was seriously flawed.

Why it mattered

The law was aimed at Moscow, and the Turkish case showed it could also reach a country that bought Russian weaponry. The S-400 purchase from Russia was the stated ground for the penalties, and the precedent was set on 14 December 2020.