In June, the Democratic-led House of Representatives finally repealed the 2001 Authorization for Use of Military Force, which passed nearly unanimously three weeks after the 9/11 terrorist attacks in the U.S. That's the AUMF that has been used for the past 18 years, giving the president the authority to attack anyone who "harbored" terrorists "in order to prevent any future acts of international terrorism against the United States."
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That "future acts" bit has been used for nearly two decades now for three presidents to justify military action all over the world, in more than three dozen military actions. This is the rationale the Trump administration will use, and will claim is legitimate under the AUMF, retroactively for the assassination of Iranian General Qassem Soleimani.
The House’s repeal, however, didn't pass the Senate. It was not included in the 2020 National Defense Authorization Act, although the House version of the conference report for it did include a repeal of the companion 2002 AUMF that authorized war with Iraq (yeah, that's still in effect) and limitations on the use of the 2001 AUMF, including an explicit prohibition of the use of military force against Iran without congressional approval.
Back in June, the Senate voted on an amendment from Democratic Sens. Tom Udall of New Mexico and Tim Kaine of Virginia to the NDAA to prevent Trump from "conduct[ing] hostilities against the Government of Iran, against the Armed Forces of Iran, or in the territory of Iran, except pursuant to an Act or a joint resolution of Congress specifically authorizing such hostilities that is enacted after the date of the enactment of this Act." Fifty senators voted for it, including four Republicans—Susan Collins (Maine), Mike Lee (Utah), Jerry Moran (Kansas), and Rand Paul (Kentucky), but it required 60 votes to pass.
Those Republicans will have another shot at this. Kaine will once again force a vote under the War Powers Resolution. Kaine has also pushed for the repeal of both the 2001 and the 2002 AUMFs, another necessity to at least attempt to curb the madman in the White House.