 A slave state since statehood in 1821, Missouri's geographic position in the center of the country and at the rural edge of the American frontier ensured that it remained a divisive battleground for competing Northern and Southern ideologies in the years preceding the war.When the war began in 1861, it became clear that control of the Mississippi River and the burgeoning economic hub of St. Louis would make Missouri a strategic territory in the Trans-Mississippi Theater.By the end of the war in 1865, nearly 110,000 Missourians had served in the Union Army and at least 30,000 in the Confederate Army; many had also fought with bands of pro–Confederate partisans known as "bushwhackers".