Historians have debated whether the Confederacy could have won the war. Most scholars emphasize that the Union held an insurmountable long-term advantage over the Confederacy in terms of industrial strength and population. Confederate actions, they argue, only delayed defeat. Civil War historian Shelby Foote expressed this view succinctly: "I think that the North fought that war with one hand behind its back...If there had been more Southern victories, and a lot more, the North simply would have brought that other hand out from behind its back. I don't think the South ever had a chance to win that War." The Confederacy sought to win independence by out-lasting Lincoln; however, after Atlanta fell and Lincoln defeated McClellan in the election of 1864, all hope for a political victory for the South ended. At that point, Lincoln had succeeded in getting the support of the border states, War Democrats, emancipated slaves, Britain, and France. By defeating the Democrats and McClellan, he also defeated the Copperheads and their peace platform. Lincoln had found military leaders like Grant and Sherman who would press the Union's numerical advantage in battle over the Confederate Armies. Generals who did not shy from bloodshed won the war, and from the end of 1864 onward there was no hope for the South.

On the other hand, James McPherson has argued that the North’s advantage in population and resources made Northern victory likely, but not inevitable. Confederates did not need to invade and hold enemy territory to win, but only needed to fight a defensive war to convince the North that the cost of winning was too high. The North needed to conquer and hold vast stretches of enemy territory and defeat Confederate armies to win.

Also important were Lincoln's eloquence in rationalizing the national purpose and his skill in keeping the border states committed to the Union cause. Although Lincoln's approach to emancipation was slow, the Emancipation Proclamation was an effective use of the President's war powers.

The Confederate government failed in its attempt to get Europe involved in the war militarily, particularly the United Kingdom and France. Southern leaders needed to get European powers to help break up the blockade the Union had created around the Southern ports and cities. Lincoln's naval blockade was 95% effective at stopping trade goods; as a result, imports and exports to the South declined significantly. The abundance of European cotton and the United Kingdom's hostility to the institution of slavery, along with Lincoln's Atlantic and Gulf of Mexico naval blockades, severely decreased any chance that either the United Kingdom or France would enter the war.

The more industrialized economy of the North aided in the production of arms, munitions and supplies, as well as finances and transportation. The table shows the relative advantage of the Union over the Confederate States of America (CSA) at the start of the war. The advantages widened rapidly during the war, as the Northern economy grew, and Confederate territory shrank and its economy weakened. The Union population was 22 million and the South 9 million in 1861. The Southern population included more than 3.5 million slaves and about 5.5 million whites, thus leaving the South's white population outnumbered by a ratio of more than four to one. The disparity grew as the Union controlled an increasing amount of southern territory with garrisons, and cut off the trans-Mississippi part of the Confederacy. The Union at the start controlled over 80% of the shipyards, steamships, riverboats, and the Navy. It augmented these by a massive shipbuilding program. This enabled the Union to control the river systems and to blockade the entire southern coastline. Excellent railroad links between Union cities allowed for the quick and cheap movement of troops and supplies. Transportation was much slower and more difficult in the South, which was unable to augment its much smaller rail system, repair damage, or even perform routine maintenance. The failure of Davis to maintain positive and productive relationships with state governors (especially governor Joseph E. Brown of Georgia and governor Zebulon Baird Vance of North Carolina) damaged his ability to draw on regional resources. The Confederacy's "King Cotton" misperception of the world economy led to bad diplomacy, such as the refusal to ship cotton before the blockade started. The Emancipation Proclamation enabled African-Americans, both free blacks and escaped slaves, to join the Union Army. About 190,000 volunteered, further enhancing the numerical advantage the Union armies enjoyed over the Confederates, who did not dare emulate the equivalent manpower source for fear of fundamentally undermining the legitimacy of slavery. Emancipated slaves mostly handled garrison duties, and fought numerous battles in 1864–65. European immigrants joined the Union Army in large numbers, including 177,000 born in Germany and 144,000 born in Ireland.

Reconstruction Era of the United States of America

Reconstruction began during the war (and continued to 1877) in an effort to solve the issues caused by reunion, specifically the legal status of the 11 breakaway states, the Confederate leadership, and the freedmen. Northern leaders during the war agreed that victory would require more than the end of fighting. It had to encompass the two war goals: secession had to be repudiated and all forms of slavery had to be eliminated. Lincoln and the Radical Republicans disagreed sharply on the criteria for these goals. They also disagreed on the degree of federal control that should be imposed on the South, and the process by which Southern states should be reintegrated into the Union. These disputes became central to the political debates after the Confederacy collapsed.

Results of American Civil War

Slavery for the Confederacy's 3.5 million blacks effectively ended when Union armies arrived; they were nearly all freed by the Emancipation Proclamation. Slaves in the border states and those located in some former Confederate territory occupied prior to the Emancipation Proclamation were freed by state action or (on December 18, 1865) by the Thirteenth Amendment. The full restoration of the Union was the work of a highly contentious postwar era known as Reconstruction. The war produced about 1,030,000 casualties (3% of the population), including about 620,000 soldier deaths—two-thirds by disease. The war accounted for roughly as many American deaths as all American deaths in other U.S. wars combined.

The causes of the war, the reasons for its outcome, and even the name of the war itself are subjects of lingering contention today. Based on 1860 census figures, 8% of all white males aged 13 to 43 died in the war, including 6% in the North and 18% in the South. About 56,000 soldiers died in prisons during the Civil War. One reason for the high number of battle deaths during the war was the use of Napoleonic tactics, such as charging. With the advent of more accurate rifled barrels, Minié balls and (near the end of the war for the Union army) repeating firearms such as the Spencer repeating rifle, soldiers were mowed down when standing in lines in the open. This led to the adoption of trench warfare, a style of fighting that defined the better part of World War I.