A historical region of north-central Europe including present-day northern Germany and Poland. Its ancient, Baltic-speaking inhabitants were conquered by the Teutonic Knights in the 1200s. West Prussia was ceded to Poland in 1466, and East Prussia became a Polish fief that passed to Brandenburg in 1618. Proclaimed a kingdom in 1701, Prussia became a military power under Frederick II (reigned 1740–1786). Prussia was instrumental in the unification of Germany, and in 1871 its king was declared Kaiser Wilhelm I of Germany.