The Senate, including a handful of Republicans, have defied Donald Trump by approving a measure to limit his war powers on Iran – but the president is poised to ensure it never becomes law.

A group of moderate and libertarian-minded GOP senators – Lamar Alexander of Tennessee, Bill Cassidy of Louisiana, Susan Collins of Maine, Lisa Murkowski of Alaska, Todd Young of Indiana, Mike Lee of Utah, Rand Paul of Kentucky and Jerry Moran of Kansas – ignored the president's call for the measure to be scuttled. It amounted to a rare rebuke from some members of Mr Trump's own party as members of both parties were worried either about his order to kill a top Iranian general or that his office has become too powerful.

House Speaker Nancy Pelosi told reporters on Thursday that she believes there would be ample votes in the lower chamber to pass the bill.

But despite the measure likely being headed to Mr Trump's desk, the president and Senate Majority Leader Mitch McConnell have kept enough GOP senators in line to have the votes necessary to block a veto-override vote, which would require a two-thirds majority in the upper chamber.

To understand why Mr Trump is set to kill the Senator Tim Kaine-crafted measure, consider these three reasons.

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All presidents do it

Then-Presidents George W Bush and Barack Obama resisted efforts by both chambers over the years to make changes to military force authorization measures that gave Congress' blessing for America's post-9/11 military operations and the former's Iraq conflict.

Why?

In short, no president – Republican or Democratic – wants to hand power over to Congress. Doing so raises the possibility they might be blamed if they were unable to eliminate a threat that harmed Americans or the country's interests.

"It is inevitable that Trump will veto the Kaine bill if it gets to his desk. A veto would not be surprising. Virtually every president has resisted having their decisions on the use of force constrained by the Congress," said Gordon Adams, a senior national security official in the Clinton White House.

Mr Trump's most recent predecessor, Barack Obama, often signalled a willingness to work with Congress to update the post-9/11 measure and possibly terminate the pre-Iraq war one, but his team and lawmakers were unable to agree on simple definitions and timelines. Each effort slowly died.

The Trump factor

Since being acquitted in the Senate, Mr Trump has fired two administration officials who offered damning testimony in the House impeachment inquiry and appeared to intervene in a Justice Department sentencing recommendation for his longtime friend Roger Stone.

Several Republican senators who opined after voting to clear him on two impeachment articles that he had learned his "lesson" this week have said they have their doubts he did so. Democrats have called for new investigations and warned Mr Trump was emboldened by most GOP senators' votes to acquit him.

Former Secretary of State Hillary Clinton, whom Mr Trump defeated in 2016 when she was the Democratic presidential nominee, went so far as to tweet that he is acting like a "tyrant" since the Senate's vote.

"This president wants to be less constrained than any previous one," Mr Adams said.

The 45th commander in chief contended in a Wednesday afternoon tweet that his foreign policy objectives would be undermined by the Kaine measure.

"We are doing very well with Iran and this is not the time to show weakness. Americans overwhelmingly support our attack on terrorist Soleimani...." Mr Trump tweeted ahead of a procedural vote on the measure, referring to Maj Gen Qassem Soleimani, head of the Iranian Revolutionary Guard's Quds Force he ordered killed earlier this year. (The Trump administration has deemed the IRG a terrorist organisation, which was part of their argument he has ample legal authorities to order such strikes -- echoing the Bush 43 and Obama administrations.)

"...If my hands were tied, Iran would have a field day," Mr Trump wrote in a second tweet. "Sends a very bad signal. The Democrats are only doing this as an attempt to embarrass the Republican Party. Don't let it happen!"

Blame game

Lawmakers would have another move after an expected veto: Both chambers could vote to override it.

But that would mean, if Mr Trump or a future president wanted to launch military operations on Iran, they would have to first approve. And that would give them ownership.

"Presidents are happy to get congressional support for uses for force. They ... don't like to ask for permission and are generally unwilling to withdraw forces at the request of Congress," Mr Adams said.