The primary cause of the War of 1812 in the U.S. was British interference with American shipping and trade. Tensions escalated due to Britain's imposition of trade restrictions, impressment of American sailors into the Royal Navy, and support for Native American resistance against U.S. expansion into the Northwest Territory. These actions sparked a desire within the U.S. for sovereignty and respect on the international stage, leading to the declaration of war in 1812. The conflict tested American nationalism and military capability, ultimately fostering a sense of unity and identity that would shape the nation in the years following the war.