The proposition that Congress had the power to initiate all wars except to repel attack on the United States is also strengthened in view of the second part of the war clause. That is the power to "grant Letters of Marque and Reprisal."
Now, most Americans, I daresay most Members of Congress, I daresay most members of Government, do not even know what the "power to grant Letters of Marque and Reprisal" means and why it is in the Constitution. An anachronism today, letters of marque and reprisals were licenses issued by governments, usually to private citizens, but on occasion to government agents, empowering these private citizens or government agents to seize enemy ships or take action on land, short of all-out war.
In essence, it was the 18th century version of what we now regard as limited war or police actions. That is what letters of marque and reprisal were. If you are having trouble with pirates off the coast, you are not looking to declare war. The Federal Government, in this case the Congress, could go out and hire out, give permission to, give a letter of marque and reprisal to a local. Think of it in terms of a local security agency that comes by and patrols your neighborhoods. You could give letters of marque or reprisal and say, "You are authorized under the law, through the Congress, to go seize those pirate ships."
That is what it was about. A leading commentator of the day-that is, the late 1700's-a leading commentator of the day on international law explained the distinction this way: "A perfect war is that which entirely interrupts the tranquility of the state. An imperfect war, on the contrary, is that which does not entirely interrupt the peace.Reprisals are that imperfect kind of war."
So, when we hear people talk about imperfect wars, it is used as a term of art as it was used back in the late 1700's. The framers undoubtedly knew that reprisals or imperfect wars could lead to general or all-out wars. England, for example, had fought five wars between 1652 and 1756 which were preceded by public naval reprisals.