President Donald Trump has told lawmakers it is not yet possible to know the "full scope and duration" of the US operation in Iran.
In a letter notifying Congress of the military action, Trump said the threat from Iran had been "untenable" despite efforts to find a diplomatic solution.
Lawmakers are set to vote this week on a war powers resolution aimed at limiting Trump's military operations in Iran. But it faces significant hurdles, and it is unclear how much its passage would alter the conflict.
The US and Israel began striking Iran on Saturday. Iran responded by firing missiles and drones at Middle Eastern nations allied to the US.
In the letter, Trump told Senate President Pro Tempore Chuck Grassley his administration's view that Iran "remains one of the largest, if not the largest, state-sponsors of terrorism in the world".
