After the Civil War, Ulysses S. Grant spent four years as head of the United States Army in peacetime. With his defeat of Robert E. Lee and the Confederacy, Grant was the most popular man in the country. As the Civil War ended Grant turned his attention to the Plains in the American West where there was numerous conflicts between white settlers, railroads, and Native Americans that resulted in wars between the Natives and the U.S. military. While the attempted Fenian invasion of Canada and French intervention in Mexico took some of his time, Grant's biggest focus was on Reconstruction of the defeated Southern states. Grant found himself caught between President Andrew Johnson, who wanted leniency to the South and a continuation of the social structure there, and the Radical Republicans in Congress, who wanted harsher punishment to rebel leaders and more government assistance to the freed slaves. Grant preserved his popularity and authority throughout the crisis that culminated in Johnson's impeachment. In 1868, the Republicans nominated Grant for president, and he won easily over his Democratic challenger.