An interesting quote I ran into in the Washington Post:

"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 [emphasis added]. . . . If, today, 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 don't.' " Abraham Lincoln February 1848 explaining his opposition to the Mexican-American War.

I found it in a guest editorial by Arthur Schlesinger Jr. about George W. Bush's war power. http://www.washingtonpost.com/wp-dyn/content/article/2006/04/23/AR2006042301014.html