President Obama spoke very well, as he often does, when he went before the United Nations to address the slaughter by the Islamic State of Iraq and Syria. “No God condones this terror. No grievance justifies these actions,” he declared. “There can be no reasoning — no negotiation — with this brand of evil. The only language understood by killers like this is the language of force.”

The president is right as rain about that, but am I the only one who thinks the president made a mistake by going to the United Nations before going to the US Congress? American jets were barely airborne on their way to Syria when editorial writers of The New York Times signaled what will no doubt be a long campaign by American leftists against the military strikes on ISIS. They — and some on the right — dispute the idea that the authorizations for the use of military force enacted in 2001 and 2002 are enough. They say those resolutions fail to cover Syria.

My view is slightly different. Either one or both of the authorizations to use military force give the president plenty of authority to go after ISIS, from Syria to the South Pole. Once war has been levied against us — as it has been by ISIS, which has beheaded two Americans and vows to attack here — my own view is that the president isn’t constitutionally entitled to fail to act. He’s sworn to defend the nation. Yet the smart move is to go to the Congress and get a proper war declaration.

The point is not to secure the president’s authority (he’s commander in chief already). It’s to bind Congress itself to the fight — no matter what the peace camp says. This is the nature of war declarations. Congress has passed them in five wars in our 238 years of independence. The formal declarations of war have been brief documents, a few sentences. What they did was acknowledge, or formally recognize, a state of war that already existed.

That’s what happened with Britain in 1812, Mexico in 1846, Spain in 1898, Germany in 1917 and both Germany and Japan in 1941. (Technically, there were separate declarations against Austria-Hungary in 1917 and Bulgaria, Romania and Hungary in 1942.)

Not only did the war declarations take formal cognizance of an existing state of war, but they pre-committed the resources of the nation to the fight. The declaration against Britain in 1812 authorized the president “to use the whole land and naval force of the United States.” It also authorized the president to issue to private armed vessels “letters of marque and general reprisal.”

Such letters — fearsome instruments — are one of the basic powers of war enshrined in our Constitution. They could be ideal in the current war (they certainly worked wonders against the Barbary pirates). The war declaration against Spain, passed in 1898, not only “empowered” but also “directed” the president to use “the entire land and naval forces of the United States.” And to “such extent as may be necessary” to “call into actual service the militia of the several states.”

What marks these war declarations is the totality of the national commitment they embody. In 1917 the Congress actually declared, “To bring the conflict to a successful termination, all the resources of the country are hereby pledged.” It made clear, in other words, that there wasn’t going to be any of what later became John Kerry’s signature move of voting against supplying our troops after he’d voted for the war. A war declaration makes clear that we’re all in.

Is this clear in the case of ISIS? Obama may have gone to the United Nations and vowed that there can be no negotiating with ISIS, but the war won’t be won in Turtle Bay. Debate will be festering on Capitol Hill throughout the fight, egged on by the ilk of the Times.

This is not a partisan question. When President George H.W. Bush was preparing to go to ground in Desert Storm, I argued that the best move was to have Congress declare. The only wars America has ever lost were undeclared. Every war that Congress did declare, we won.

This column first appeared in the New York Post.