The Iran War Powers Resolution passed both Houses because some Republicans, seizing an opportunity to oppose the decades-long expansion of the president's war powers, voted for it - including, in the Senate, Sens.
In a formal statement released by the White House, Trump called the measure a "very insulting resolution" that Democrats introduced as a wedge issue to divide Republicans ahead of the November elections.
Those efforts gained renewed urgency in January when Trump ordered the killing of Qassem Soleimani, the commander of Iran's elite Quds Force.
The U.S. Central Command said earlier this week it has awarded 29 soldiers the Purple Heart medal that is given to those killed or wounded in action. Soleimani, who led the Iranian Revolutionary Guard Corps, directed local militias to storm the USA embassy in Baghdad.
The move marks the seventh time Trump has used his veto pen, including on a previous war powers resolution related to the U.S. military's assistance to Saudi Arabia in Yemen's civil war.
The Senate may vote to override the president's veto, thought at this point it's doubtful there will be enough votes to do so.
Trump's veto came two weeks after the president said he instructed the U.S. Navy to "shoot down and destroy" any Iranian vessels that "harass" American ships in the Persian Gulf, further heightening the possibility of all out war.
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In the statement, Mr. Trump said the Congress misinterpreted his constitutional authority as being limited to "defence of the USA and its forces against imminent attack". "We live in a hostile world of evolving threats, and the Constitution recognizes that the President must be able to anticipate our adversaries' next moves and take swift and decisive action in response". But the actual transmission of the congressional resolution to the White House was delayed for almost two months as Congress left Washington due to the coronavirus pandemic, according to aides.
It is the second time Mr. Trump has vetoed an attempt by Congress to control his military initiatives.
"This resolution is about Congress reclaiming its rightful role in decisions about war", Sen. "That's what I did!", Trump said, seemingly referring to the Soleimani's assassination. "Congress should seek to pass this resolution again and overcome the presidential veto, whether by overriding or inserting war prohibitions into a must-pass bill later this year".
"I urge my colleagues to join me in voting to override his veto - Congress must vote before sending our troops into harm's way", he tweeted. An offensive war requires a congressional debate and vote.
"The President and his allies falsely claimed that attacking Iran was necessary to stop an imminent threat ..."
"Vetoing this legislation does nothing to establish congressional authorisation for the President's actions".
The resolution said the U.S. President can not commit his military to hostilities against Iran or any part of its government or military without explicit authorisation from the Congress.