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”.
He said it “continues to seek the means to possess and employ nuclear weapons”.
“Its array of ballistic, cruise, anti-ship and other missiles pose a direct threat to and are attacking US forces, commercial vessels and civilians, as well as those of our allies and partners,” the letter added.
The president also said no US ground forces have been committed to the operation and that it was designed to minimise civilian casualties.
Democratic lawmakers – and a handful of Republicans – have questioned whether Trump violated the law by launching strikes without congressional approval. Many have supported efforts to limit Trump’s ability to take further action in Iran.
While the president has broad authority to launch military action without a formal declaration of war, Congress by law has to be notified within 48 hours of hostilities beginning.
“The constitution says we’re not supposed to be at war without a vote of Congress,” Virginia Democratic Senator Tim Kaine, a co-sponsor of the war resolution, told NPR.
“This is important. The lives of our troops are at risk,” he added. “We ought to come back to Washington right away and vote on this.”