WASHINGTON — The White House told Congress on Friday that President Trump authorized the strike last month that killed Iran’s most important general to respond to attacks that had already taken place and deter future ones, contradicting the president’s claim that he acted in response to an imminent threat.

In a legally mandated, two-page unclassified memo to lawmakers, the White House asserted that the strike that killed Maj. Gen. Qassim Suleimani was “in response to an escalating series of attacks in preceding months” by Iran and Iran-backed militias.

“The purposes of this action were to protect United States personnel, to deter Iran from conducting or supporting further attacks against United States forces and interests, to degrade Iran’s and Quds Force-backed militias’s ability to conduct attacks, and to end Iran’s strategic escalation of attacks,” said the report, which was transmitted on Friday to the House Foreign Affairs Committee.

The document confirmed what lawmakers had privately suspected as the Trump administration has offered a shifting set of justifications for the strike against General Suleimani in Baghdad — taken with no congressional consultation — which brought the United States and Iran to the brink of war.