So now President Donald Trump is a war leader. His decision to launch 59 Tomahawk cruise missiles across an international border is an act of war. And, like most of America's wars, it will never be declared by Congress.

Immediately upon the news of the Syria strike breaking Thursday night, Twitter erupted with complaints that the Constitution vests the power to declare war only in Congress. This common worry misapprehends both the structure of the Constitution and the historical understanding of the declaration of war.

Let's start with the obvious: Every U.S. president, all the way back to the founding, has at some point used military force without first obtaining the approval of the legislative branch.

A few snippets: George Washington fought the so-called Northwest Indian War to subdue the native people of Ohio. James Monroe sent forces to conquer Amelia Island, off Florida. James Buchanan sent Marines to halt a civil war in Nicaragua. In 1893, U.S. forces overthrew the government of Hawaii, although apparently without White House permission. Still, the overthrow stuck.

On the eve of World War I, Woodrow Wilson ordered the Marines into Mexico. Half a century later, Ronald Reagan invaded Grenada. Most prominently, in the Cuban missile crisis, President John Kennedy took the nation to the brink of nuclear war with the Soviet Union.

More recent history is much the same. In 2011, the White House justified President Barack Obama's orders to attack Libya with the remarkable argument that because U.S. forces were conducting only bombing and using missiles, the actions did not constitute "hostilities" within the meaning of the War Powers Resolution of 1973 — a statute requiring that hostilities end within 90 days if no congressional approval is forthcoming.

True, presidents often claim to find justification for their wars in the language of existing statutes and resolutions. Obama relied regularly on the Authorization for Use of Military Force adopted by the Congress after the Sept. 11 terrorist attacks. That resolution is now more than a decade and a half old, but I have no doubt that the Trump White House will soon be citing it as legal authority.

But what about the congressional power to declare war? Scholars nowadays are sharply divided over whether the framers intended it as a check on the executive's "independent" warmaking authority. Certainly at the time of the founding, the use of a formal declaration of war had fallen into desuetude. Yes, there is a reasonable case to be made that the framers did indeed hope to restrict presidential use of the military without congressional assent. (That's one reason for the early resistance to a standing army.) But the new nation did not behave as if a declaration was necessary. (Abraham Lincoln's famous aphorism that the country should never go to war through the will of one man was a response to President James Polk's invasion of Mexico somewhat in advance of congressional permission.)

Even 19th- and early 20th-century thinkers who considered a declaration of war to be important did not argue that it was constitutionally required. The reason a declaration should be made, wrote the prominent lawyer Daniel Chauncey Brewer in the 1890s, was that without it "neither enemies, friends, nor neutrals can be properly forewarned." The function of a declaration of war, wrote Yale's Simeon Baldwin as World War I erupted, was to create or announce the "legal condition of things."

It's because of the need to give notice of hostilities that the Dutch lawyer and philosopher Grotius, in his 17th-century treatise, condemned the use of force without formal declaration of war. By the early 20th century, largely as a result of actual practice, the treatises on international law had largely reduced this notion to a requirement that hostilities be preceded by either a declaration or an active controversy between the belligerents. In both cases the worry was about surprise attack. The declaration of war gave formal warning that diplomatic means had been exhausted, so that both enemies and neutrals should now be prepared for the worst.

But that need for formal warning is the relic of an era of more difficult communications. Trump and Obama both warned Syria. President George W. Bush warned both Afghanistan and Iraq. The world braced for action. The warning function, in short, is nowadays carried out by less formal but still effective means.

I have argued that the declaration of war clause in the Constitution represents an effort to limit the president's ability to undertake an "offensive" war — what 18th-century tradition would have understood as a war for conquest or glory. Perhaps you find that reading too narrow. Still, the lessons of Supreme Court Justice Felix Frankfurter hold: In the end, the meaning of the Constitution is largely determined by how the political branches of the federal government act over time. And over time, the power to declare war has become a dead letter.

Still, the Congress possesses considerable powers to prevent the president from making war should it choose to exercise them. In particular, the House and the Senate can vote to cut off funding, as they did in 1973, overriding a presidential veto of a law refusing to pay for the bombing of Cambodia.

That's tough to do, of course, but it would be a lot easier if Congress had not acquiesced over the years in the buildup of warmaking authority in the executive.

There's zero chance that the House or Senate would adopt a resolution restricting Trump's freedom to act in Syria, just as there was zero chance that either chamber would adopt a resolution restricting Obama's freedom to act in Libya.

The result is that the commander in chief can order the U.S. military into action whenever it suits his judgment. Many people, including me, are uneasy with that hard truth. For better or worse, however, it's been our practice for a very long time. Clinging to the long-dead notion that Congress must first declare war might be comforting, but it has nothing to do with reality.

Stephen Carter is a professor of law at Yale University and was a clerk to U.S. Supreme Court Justice Thurgood Marshall. His novels include "The Emperor of Ocean Park" and "Back Channel," and his nonfiction includes "Civility" and "Integrity."