Lincoln is often praised for his work as a wartime leader;

his public statements, most notably the Gettysburg Address, defined the war issues and helped redefine America's self image. He proved adept at replacing mediocre generals with better ones until he finally found a winner in Ulysses S. Grant. In leading the Republican party he kept all factions together and added new support from War Democrats, even as his Copperhead enemies were lambasting him as a ruthless dictator. Lincoln had to negotiate between Radical and Moderate Republican leaders, who were often far apart on issues of slavery.

He personally directed the war effort, in close cooperation (1864-65) with General Grant, who in April 1865 accepted the surrender of Robert E. Lee's main army.His leadership qualities were evident in his first diplomatic handling of the border slave states at the beginning of the fighting, in his defeat of a congressional attempt to reorganize his cabinet in 1862, in his many speeches and writings which helped mobilize and inspire the North, and in his defusing of the peace issue in the 1864 presidential campaign. Copperheads criticized him for violating the Constitution, overstepping the bounds of executive power, refusing to compromise on slavery, declaring martial law, suspending habeas corpus, ordering the arrest of 18,000 opponents including public officials and newspaper publishers, and killing hundreds of thousands of young men who were soldiers in the war.

Radical Republicans criticized him for moving too slowly in abolishing slavery, and not being ruthless enough toward the conquered South.Lincoln is most famous for his roles in preserving the Union and ending slavery in the United States with the Emancipation Proclamation and the Thirteenth Amendment to the United States Constitution.