In November 1806, Napoleon ordered that all European ports under his control be closed to British ships. He later extended this policy to neutral ships who have entered a British port before arriving at the continent. Britain replied with a series of Orders in Council. These required all neutral ships to acquire a license in a British port before they could sail to Europe.
This contradiction meant that all neutrals would have to choose whose orders they were to follow. Britain had a more powerful navy, but American economy depended on trade with Europe, and there had been instances of high-level government contact between France and the United States.
By 1812 British ships had captured almost four hundred American vessels, some within sight of the U.S. coast, severely disrupting with American export trade. Often, Americans were conscripted to serve on British ships. As a result, many Americans called the War of 1812, the "second war for independence". John Clopton of Virginia wrote:
"The outrages in impressing American seamen exceed all manner of description. Indeed, the whole system of aggression now is such that the real question between Great Britain and the United States has ceased to be a question erely relating to certain rights of commerce ... it is now clearly, positively, and directly a question of independence, that is to say, whether the United States are really and independent nation."
American sailors are removed from their ships by the British
Source: The War of 1812, by Peter I. Bosco
The United States army was not well fit to conquer Canada in 1812. There were few regular troops, but mostly militia of fighting-age male citizens, commanded by inexperienced officers. There were disagreements between the states on whether militamen should fight outside their states (for militia, unlike regulars, were not under the direct command of the president). Other states did not feel threatened, and were unwilling to take part in the war.
Canada, however, had even less military forces. Sir George Prevost, Governor of the Upper Canada, estimated in May 1812 that only 4,000 of Upper Canada's 11,000 militia could be relied upon to be loyal. Britain was unable to help, its resources consumed by the war with Napoleon.
Despite this, on June 18, 1812, Congress passed a bill, seventy-nine to forty-nine in House of Representatives, nineteen to thirteen in Senate, approving President Madison's declaration of war. Britain tried to delay its own declaration, in a futile hope that war could be averted, until January 9, 1813, but military action started a lot earlier.