For at least the past ten years American Presidents, including Barack Obama, have been not-declaring their wars, including their secret wars. What’s wrong with this situation? Glenn Greenwald quotes none other than Abe Lincoln:

There are few things more dangerous in a democracy than allowing a President to wage secret wars without the knowledge of the country. I’ll permit Abraham Lincoln — not exactly a pacifistic worshipper of legalisms and restraints on Executive power — to explain why this is so, in an 1848 letter to a proponent of unrestrained presidential warmaking powers:

Allow the President to invade a neighboring nation, whenever he shall deem it necessary to repel an invasion, and you allow him to do so, whenever he may choose to say he deems it necessary for such purpose and you allow him to make war at pleasure. Study to see if you can fix any limit to his power in this respect, after you have given him so much as you propose.

If, to-day, he should choose to say he thinks it necessary to invade Canada, to prevent the British from invading us, how could you stop him? You may say to him, I see no probability of the British invading us but he will say to you be silent; I see it, if you dont.

The provision of the Constitution giving the war-making power to Congress, was dictated, as I understand it, by the following reasons. Kings had always been involving and impoverishing their people in wars, pretending generally, if not always, that the good of the people was the object. This, our Convention understood to be the most oppressive of all Kingly oppressions; and they resolved to so frame the Constitution that no one man should hold the power of bringing this oppression upon us. But your view destroys the whole matter, and places our President where kings have always stood.