Article I, Section Eight of the U.S. Constitution gives Congress the sole power to declare war, and it was the first time the legislative body had exercised that power. But it was an extremely close vote: Madison’s party, the Democratic-Republicans, was divided over the prospect of starting a war with a global superpower like Great Britain. Across the aisle, the rival Federalist party was uniformly against the idea.
Federalists dominated New England, whose seafaring communities depended on trade with the British. (The party also had some strong reservations about France’s government and its leader, Napoleon Bonaparte.) So when the Madison-backed war resolution came up for a vote in Congress, not a single Federalist supported it. The measure passed anyway: On June 4, the House of Representatives voted 79 to 49 in favor of going to war. The Senate responded in kind on June 17, with 19 yes votes and 13 nos. No other declared war in U.S. history has ever been approved by such a narrow margin in Congress.