It's unclear — perhaps even to President Trump himself — whether the cruise missile attack he ordered on an air base in Syria last week will be followed by other military action against the government of President Bashar Assad. What is clear is that Trump, like so many of his recent predecessors, is perfectly comfortable using military force without authorization from Congress.
That was evident not only in last week's attack, which was designed to punish Assad for using chemical weapons against his own people, but also in Trump's continuation, and in some ways intensification, of Barack Obama's campaign to "degrade and ultimately destroy" Islamic State in both Syria and Iraq. In neither case did Trump seek congressional approval.
In a letter sent to Congress after last week's attack, Trump said he had acted "pursuant to my constitutional authority to conduct foreign relations and as commander in chief and chief executive." His only concession to congressional authority was to say that he was informing Congress "consistent with" — not in compliance with — the reporting requirements of the 1973 War Powers Resolution, a law that attempted to rein in presidential war making.
There is a built-in tension in the Constitution between Congress' power to "declare war" and the president's responsibility as commander in chief to protect the nation and its armed forces. Sometimes the president will need to deploy the military to respond to emergencies without seeking prior congressional approval.
But it's not clear that last week's attack satisfied that exception. In 2013, Obama, when he was considering similar action to punish Assad for using chemical weapons, agreed to seek Congress' permission. (He abandoned the military option after Syria agreed to dismantle its chemical weapons program, an undertaking it obviously has reneged on.)
Obama also sought congressional approval for the war against Islamic State, a much more sustained campaign. But he undermined the urgency of his request by insisting implausibly that he already had all the legal authority he needed under two authorizations for use of military force approved years earlier for different purposes. One, passed in 2001, was aimed at the perpetrators of 9/11. The other, approved in 2002, was designed to deal with "the continuing threat posed by Iraq" — the Iraq of Saddam Hussein.
Those were flimsy legal justifications in the Obama administration and they are still flimsy. Sustained military campaigns — whether the objective is defeating Islamic State or effecting regime change in Damascus — require timely and specific congressional approval. That's true regardless of who occupies the White House.