President Trump said he was against forever wars in his State of Union: “Great nations do not fight endless wars.” He also made a point of shifting U.S. policy in the Middle East, calling for withdrawals from both Afghanistan and Syria — even over the objections of his then-Defense Secretary Jim Mattis who would resign over the disagreement.

But Trump was not against forever wars on Tuesday. When presented with bipartisan legislation to end U.S. involvement in Yemen evoking the War Powers Act, he vetoed it, arguing that forever wars are within his constitutional power.

As he put it in a letter to the Senate, “This resolution is an unnecessary, dangerous attempt to weaken my constitutional authorities, endangering the lives of American citizens and brave service members, both today in the future.”

Of course, it’s unclear just where Trump thinks his constitutional authority to engage in conflicts abroad comes from. Article I, Section 8 of the Constitution, after all, makes it clear that only Congress has the power to declare war. The president, instead, has the power to direct the military to engage only after such a declaration.

Supporters of U.S. involvement in Yemen, however, have argued that the U.S. isn’t “technically” engaged in the conflict because it only provides logistical military support and, of course, authorizes billions of dollars in arms sales to the Saudi-led coalition.

But such shadowy activities are directly linked to the conflict even if there are no ground troops, and they have real consequences. The four-year civil war, waged with U.S. support, has caused a humanitarian crisis, left thousands of civilians dead — some at the hands to the very airstrikes the U.S. was assisting with — and failed to produce meaningful steps toward peace.

That seems to be exactly the sort of involvement that the founders intended to leave in the hands of Congress. As Alexander Hamilton explained in "Federalist No. 69" in 1788:

The President is to be commander-in-chief of the army and navy of the United States. In this respect his authority would be nominally the same with that of the king of Great Britain, but in substance much inferior to it. It would amount to nothing more than the supreme command and direction of the military and naval forces, as first General and admiral of the Confederacy; while that of the British king extends to the DECLARING of war and to the RAISING and REGULATING of fleets and armies, all which, by the Constitution under consideration, would appertain to the legislature.

The Constitution explicitly delegates to Congress the power to decide which conflicts the U.S. engages. By the founders' design, the executive can only act after lawmakers make their decision. Presidents do not have the sole discretion to engage in military operations.

If Trump is serious about reconsidering U.S. engagement abroad and preventing the endless conflicts he has warned about, that interest cannot be limited simply to the areas where he, through indifference, misunderstanding or design, finds it useful. It also cannot be his decision alone.

The one thing that Trump is right on is that an end to forever wars would be a limit on his power. But that's a limit written into the Constitution, and one that lawmakers have neglected to enforce.