The main cause of the War of 1812 between the U.S. and Britain was largely due to maritime issues, particularly British impressment of American sailors and interference with American trade. Britain was involved in ongoing conflicts with France at the time and sought to control American shipping routes, which disrupted U.S. commerce. Additionally, there were heightened tensions over British support for Native American resistance against American expansion into the Northwest Territory. These grievances culminated in President James Madison declaring war against Britain in 1812, marking a significant conflict in U.S. early history.