At the dawn of 1812 the United States was heading inexorably towards war. The United States had a list of complaints against the British; from the continued impressment of its sailors, the seizing of its ships, and the belief the British were fomenting Indian rebellions on the Northwest frontier. All of these were sufficient reasons to go to war. In addition, there was always a contingent of colonists who wanted to attempt to conquer Canada.
The problem was that the United States Army and Navy were not up to the task of fighting the British. At the start of 1812 the US Army had only 4,000 officers and men. At that same time the Navy consisted of five frigates, three sloops and seven brigs. However, Britain was engaged in a World War with France under Napoleon. As a result British forces were stretched rather thin.
In January 1812, Congress authorized the growth of the US Army to 30,000 officers and men. Merely authorizing an increase in soldiers was not sufficient to grow the army. So then Congress further authorized 30,000 one-year volunteers, which would include members of state militias. War was on the horizon. However, there was a clear lack of enthusiasm. Other than a discreet group of war hawks focused on attacking Canada, no one really wanted to go to war. There was also the question of whom to go to war with. Many were more interested in going to war with France than with Britain. President Madison sent a letter to Congress outlining the fact that since November 1807 the British had seized 389 American vessels, while the French had seized 558 during that same period of time. It was only the British, however, that impressed sailors. Madison was reluctant to ask the country to go to war. Finally, on June 1st, 1812, he sent a request to declare war on Great Britain to the Congress.
The House, and then the Senate, debated the declaration for half the month. Finally, on June 17th, following an earlier vote by the House the Senate approved the Declaration of War by a vote of 19 to 13. Some Senators who had opposed the vote earlier, chose in the end to vote in favor. So by a small margin, and with little enthusiasm, Congress voted a Declaration of War against Great Britain.