How Few Remain

How Few Remain, A Novel of the Second War Between the States is an alternate history novel written by Harry Turtledove and winner of the Sidewise Award.

Warning: Wikipedia contains spoilers

Its point of divergence is 1862, in which a General Order of Robert E. Lee's was not found by General George McClellan and thus the armies of the Confederate States of America were finally victorious over the United States and won international recognition from Great Britain and France.

Forward to 1881, where the Confederate States of America has purchased two states from Mexico and becomes the instigator of frequent border raids into United States territory. The U.S. has allowed its military to fall into disrepair after its loss of the Civil War (called the War of Secession in the Confederate States); Abraham Lincoln lost the 1864 election and attempts to rebuild the Republican party, but slavery is its stumbling block to unification. Lincoln forms links with the Socialist Party while Theodore Roosevelt ranching in the Dakota Territory prepares with General George Custer to repel a British invasion from Canada. Just before they cross the border, President James Blaine of the United States and President "Stonewall" Jackson of the Confederate States sign a treaty to cease fighting, with a Confederate victory in the Battle of Louisville.