The U.S. declared war on Britain June 18, 1812. Transatlantic communication took 3–6 weeks—so early operations unfolded before British authorities in North America received official word.
Pre-war. Harrison defeated Tecumseh's confederacy at Prophetstown. Not a War of 1812 battle but often conflated. The question likely references this.
Harrison's Kentucky militia destroys Miami villages in Indiana Territory — preemptive frontier security.
Britain's Orders in Council (trade restrictions) were repealed June 23, 1812 — after U.S. declaration but before news reached Washington. Had communication been instant, war might have been averted. Instead, the "Second War of Independence" began in ignorance.