Coming out of the American Revolution, the US and Britain still faced many unresolved tensions. The British practice impressment had gone on for hundreds of years, and the independence-minded Americans were no longer willing to tolerate such an affront to their sovereignty. Further, thanks to memories of the Revolutionary War, deep enmity remained between the two nations: Britian had a score to settle, and the US felt deeply that it had to stand its ground. Many at the time of the War of 1812 considered it a "Second War for American Independence," linkages with the first stood strongly in many minds.
While pressure for war with England built during his presidency, Thomas Jefferson looked back to the painful struggle of the Revolutionary years and did all he could to search for more peaceful alternatives, such as an embargo. The legacy of the Revolutionary War, however, had a double impact. While it discouraged the desire for war in the generation that fought it, its successes made the men of the next generation seek a war of their own. It was a desire to mathc and surpass their father's tales of the Revolutionary War that made the young War Hawks in Congress so pro-war by 1812.
Most clearly, the War of 1812 was an outgrowth of the Napoleonic Wars, a series of conflicts in Europe from around 1800 to 1815. Napoleon was trying to hurt Britain's economy with his Continental System; Britain retaliated with economic sanctions of its own, preventing imports from the rest of the world into European ports. Fledgling America felt the pinch in this titanic battle between France and Britain, and it made an attempt to play the great powers against each other with Macon's Bill No. 2, which promised US support for whichever nation stopped the embargo on American ships. These attempts backfired, as Napoleon tricked Madison by agreeing to abide by Macon's bill without actually doing so. The US had taken a powerfully antagonistic position toward Britain with little to show for it. The situation fell quickly into war.
The War of 1812 had several important long-term results for the US. First, because it involved a necessary ban on British manufactures, the war sheltered New England factory owners. Without having to compete with cheap British goods, American industry jump-started during the war years, accelerating the pace of industrialization in the North, a process that would continue for the next hundred years with few interruptions. Also, at the end of the War of 1812, Henry Clay proposed an "American System," including the building of a national railroad network. The project of building a national railroad would continue on through the 19th century, transforming the nation in the process.
Second, at the end of the war, Federalists upset with the war met at the Hartford Convention. The meeting appeared to the country as unpatriotic at best, and treasonous at worst. As a result, Federalist power declined rapidly, and the Federalists ceased to be players on the national stage. At the same time, the mild Federalist talk of secession at the Convention helped provide a foudnation for theories of States' Rights and secession that exploded in the Civil War.
Finally, the War of 1812 produced many heroes. In terms of future American politics, eventual presidents Andrew Jackson and William Henry Harrison rose to national prominence by battling Indians and Redcoats during the war. America's conception of itself also gained a defining gem duyring the war: Francis Scott Key composed the words to the "Star Spangled Banner" at the Battle of Baltimore in 1814. Further, this second war against Britain boosted American nationalism, and paved the way for a primarily isolationist 19th century America. And though it would be another century before the US would emerge as a world power, after going once again facing the British and emerging intact, US sovereignty in the Western Hemisphere would never again be legitimately challenged.