Forum:Longer Civil War

What would happen if the civil war was much longer, like 10 years? What would its effect be?

Here are the circumstances:

In the Mexican-American war, the US annexed the OTL Mexican cession as well as the Mexican states of Baja(all of it), Sonora,Chihuahua,and the states along the Rio Grande. During the Civil War, they revolted and fought alongside the CSA for independence in order to later join back up with Mexico.

Robert E. Lee is with the Union Army.

If this was a protracted war like this, how would it affect the US?