During the American Revolution, support for the Revolution was strongest in New England, and eventually, support for independence from Britain was the strongest there out of any of the thirteen colonies, later U.S. states in the American Union; so why was opposition to the War of 1812 against Great Britain the strongest in New England as well?
It wasn't exactly a united front when the US declared war on Great Britain in June 1812. The reasons given for going to war officially were maritime rights and the impressment issue: Britain denying free trade to Europe with its massive naval blockade and seizing American sailors aboard neutral US vessels to press them into Royal Navy service, during its titanic struggle against Napoleon.
But Congress was dominated by a group of young statesmen on the frontier, known as War Hawks, who were angered at what was perceived as Britain's meddling in Indian Territory in the Ohio Valley. British goods and even British weapons were found in native villages when US settlers began encroaching into the territory. For them, it was seen as an affront to national honour that demanded action. Tecumseh, the leader of a pan-native alliance and bitter enemy of US frontier settlers, aligned himself with Britain in hopes of securing a native homeland. This further enraged the War Hawks.
This Indian territory was meant as land for natives since the 1763 Proclamation after the Seven Years War and served as a buffer zone between British colonial possessions in Canada and the US. US settlers intent on westward expansion rejected such limitations -- they wanted unrestricted access to the region. So the war objectives of the frontier statesman and their maritime colleagues differed from the start, and this was reflected in the -- at times -- haphazard planning for the campaign.
The capture of Quebec, the political and economic heart of British North America, should have been America's top priority. Its capture could then be used as a bargaining chip in negotiations with Britain, but in 1812 this wasn't its focus. Most of the land battles in the war took place in the Niagara region of Upper Canada, a telling sign of what US intentions were from 1812-13. This pleased the War Hawks as it served their interests, but it didn't relieve the economic threats New England would soon face.
Although New England begrudgingly went along with the war initially, the economic reality was that they relied heavily on the cross-Atlantic trade with Britain and the robust trade with Britain's maritime colonies, primarily nearby Nova Scotia. They had common roots, familial ties and traded openly with both sides throughout the conflict. The war directly impacted and harmed their livelihoods.
While the US did score important ship-to-ship victories and the Battle of Lake Erie in 1813 all but ensured the end of British influence in the Ohio Valley, we need to note that fleet to fleet -- there was no contest: Britannia ruled the waves and was strangling America's cross-Atlantic trade with impunity. By 1814, the US was all but broke and New England was bearing the brunt of Britain's punitive campaign on the Atlantic seaboard, with Washington in flames and large swaths of Maine under British control. There were genuine fears of a naval assault on Boston to follow. New England was seething in opposition to a war that had become a burden and visceral threat to their prosperity, leading to the Hartford Convention and rumblings of secession. Only the war's end made the crisis a moot point.
By the time the US was prepared to face British forces on even terms in 1814 with a professional regular army -- and not the ill-trained, part-time militia it had cobbled together in 1812, Napoleon had abdicated. This allowed the British to send considerable resources with seasoned veterans of Napoleonic campaigns and finally turn what had been a defensive war (protecting Canada from invaders) into an offensive war. Late attempts by American forces to finally invade and potentially seize Quebec were repelled at Chateauguay and Crysler's Farm.
The Treaty of Ghent became a face-saving measure for both sides. Britain was weary of war but defended Canada despite the odds, while America had asserted itself on the international stage. All sides ignored the native peoples in the negotiations, the real tragedy in this conflict.
The frontier settlers gained access to the US interior, while New England quickly restored its cross-Atlantic trade relationship with Britain.