War powers

Article I, Section 8, Clause 11 of the United States Constitution, sometimes referred to as the War Powers Clause, vests in the Congress the power to declare war, in the following wording:

A number of wars have been declared under the United States Constitution, although there is some controversy as to the exact number, as the Constitution does not specify the form of such a declaration.

Five wars have been declared by Congress under their constitutional power to do so: the War of 1812, the Mexican–American War, the Spanish–American War, World War I, and World War II.

In a message to Congress on May 11, 1846, the president, James K. Polk, announced that Texas was about to become a state. Consequentially, Mexico then threatened to invade Texas, upon which the President amassed troops in the area of Corpus Christi. Texas then became a state, and US troops moved into an area in which the new international boundary was disputed. Mexican troops moved into the same area, and the two forces clashed. The President then stated "after reiterated menaces, Mexico has passed the boundary of the United States, has invaded our territory and shed American blood upon the American soil. She has proclaimed that hostilities have commenced and that the two nations are now at war." Some in Congress wondered if this were so, including Abraham Lincoln. He wrote in a letter to his law partner:

"Let me first state what I understand to be your position. It is, that if it shall become necessary, to repel invasion, the President may, without violation of the Constitution, cross the line and invade the territory of another country; and that whether such necessity exists in any given case, the President is to be the sole judge. ... But 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 allow him to make war at pleasure. … 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 don't.'

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