Operation Epic Fury is over. Or at least, that’s what the US secretary of state, Marco Rubio, announced on May 5, describing any further US action in the Gulf as purely “defensive”.

Rubio’s insistence that the conflict the US and Israel launched on February 28 achieved its objectives is open for debate. But this change of tone and terminology is likely to reflect arguments that raged in the US Congress as the war approached the two-month mark at the end of April, about whether the Trump administration must seek congressional approval for the conflict as required by US law.

The conflict has become the latest episode in a long struggle between the US Congress and the presidency over which branch of government can legitimately start wars. And, in a surprising way, Donald Trump’s actions seem to be pushing power back towards Congress.

The US constitution splits war powers between the presidency and Congress. It gives Congress the power to raise armies and declare war but makes the president the commander-in-chief of the military. That means that, in theory, you need to get Congress to agree to fund and start a war and the president to agree to wage it.

Since the second world war, this system has been changing. The last time the US formally declared war was in 1942 against Bulgaria,........

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