What was the effect of the Civil War?

Answer 1

The question of whether or not a state could legally leave the United States was decided and the answer is "no."

Slavery was abolished (although not by the Emancipation Proclamation, which only emancipated all slaves in states of rebellion aka the Confederacy. It was not until the 13th Amednment was passed that slavery was officially abolished and suffrage was granted to all black men of voting age along with citizenship for all blacks living within the US).

Many of the more commonly practiced methods of warfare were changed. Napoleonic tactics officially became obsolete. The invention and mass-production of the rifled musket made the old Napoleonic columns of the first part of the century extraordinarily dangerous and made assaulting an enemy position next to impossible without massive artillery or overwhelming numbers.

Total war was discovered (or rediscovered, depending on your stance), which stated essentially that in order to bring down an enemy state then every part of that state becomes the enemy: including civilians and non-military infrastructure.

The Gatling gun was invented.

The Civil War helped to bring the country together. Most of the soldiers who fought had never left their home counties, much less their home state, and yet here they were marching up and down a continent that they didn't know, yet all of it was their home. These long marches and campaigns, interchanges between Confederate and Union troops and even amongst the armies themselves, helped Americans to gain a sense of national identity, which had been remarkably difficult for a country of our size.