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President Donald Trump crushed a looming congressional revolt against the war in Iran last week, though the rebellion was more posed than real and would likely have petered out on its own.

The reason is that the War Powers Act, the 1973 law that slim majorities in the House and Senate invoked in a joint resolution, has rarely been taken seriously on Capitol Hill. In fact, except for the first few years after the passage of the act—which was meant to put muscle and teeth on the legislature’s responsibilities under Article 1 of the Constitution—it has never been taken seriously enough to matter.

When the act was passed, the Vietnam War was unwinding, and Richard Nixon’s presidency was collapsing in the wake of the Watergate scandal. Sentiment against the war—and against excessive executive power—was so widespread and deep that after Nixon vetoed the War Powers Act, both houses of Congress mustered the two-thirds majority to override the veto.

The following year, Nixon would resign to avoid impeachment, and Democrats would enlarge their veto-proof majorities in Congress. Soon after, they would stop the flow of money to the war in Southeast Asia, bar the use of funds to fight communist rebels in Angola, and place restraints on the CIA and the National Security Agency.

But that was pretty much it. The War Powers Act requires presidents to notify Congress when they send troops into battle. Presidents have done so 132 times; each time, Congress has assented or at least not objected in any tangible way. President Barack Obama went past the 60-day limit in Libya, but argued that the act didn’t apply because authority had been transferred to........

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