Final answer:
Federalists opposed the War of 1812 mostly because they believed it would harm trade, especially in maritime-dependent New England, and their discontent was evident at the Hartford Convention.
Step-by-step explanation:
Opposition of Federalists to the War of 1812
The opposition of the Federalists to the War of 1812 was primarily rooted in the belief that it would harm trade. This concern was especially pronounced in New England, where the economy was heavily reliant on maritime commerce. The negative economic impact of the conflict, combined with political discontent with leaders from the slaveholding South, like Jefferson and Madison, fueled Federalist opposition. The depth of their disapproval was evident at the Hartford Convention, which entertained the idea of secession by some attendees. After the war, which ended with the Treaty of Ghent, the Federalists' reputation suffered due to their perceived lack of patriotism during a time of conflict, ultimately leading to the party's downfall.