The killing risks further raising tensions across the Mideast, nearly a year after Iran and the U.S. stood on the brink of war when an American drone strike killed a top Iranian general in Baghdad.
With the official withdrawal from the treaty, which allowed the U.S. and Russia overflight rights to inspect military facilities, only one arms-control pact is still in force between the former Cold War foes.
President Trump will leave office with a marginally stronger military than when he entered, but he is also leaving President-elect Biden with a set of more potent threats.