During the second quarter of the 1800s, the United States expanded into new western territories. War between the United States and Mexico ended in 1848 with the United States receiving full title to Texas, California, and what was then New Mexico territory. The question of allowing slavery in the new territories, however, threatened national unity. The Compromise of 1850, which admitted California as a free state and allowed voters in the other territories to decide for or against slavery, temporarily dampened heated sectional debate.
In the election of 1860 slavery was the key issue, and the Republican candidate, Abraham Lincoln, won. Most of the Southern states, whose leaders viewed Lincoln as an abolitionist, seceded from the Union.
In the Civil War, the South relied on superior generals, while the North drew upon its advantages in manufacturing, population, transportation, and wealth. Confederate armies at first outmaneuvered Union forces in the East, but they did not succeed in invading the North. In the West, Union armies eventually gained control. Grant's capture of Vicksburg and Sherman's march to the Atlantic coast divided the South.
In 1862 President Lincoln's Emancipation Proclamation transformed the war from a conflict fought mainly to preserve the Union into a crusade to end slavery. Lincoln won reelection as president in 1864. In April 1865, the South surrendered, but the President's assassination just five days later weakened any chances for a peaceful reconciliation between North and South.
