To declare War, grant Letters of Marque and Reprisal, and make Rules concerning Captures on Land and Water;
This means that Congress and only Congress has the power to declare war, not the President. The founders knew that heads of state were far too eager to engage military conflicts. In the subsequent centuries, more and more war powers have been claimed by the executive branch, leading to a steady erosion of the force of this clause.
Of course, Congress is partly responsible for this shift. The idea of checks and balances is that occasionally a branch has to check another branch. Congress could have voted to officially declare war in Iraq, Vietnam, etc. but they didn’t, vacating a responsibility that the President took over.
Additionally, Congress controls the budget (“the power of the purse”), and they allocate the resources to fight battles launched by the Executive Branch. Don’t like a war? Don’t fund it.
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