In the course of the us civil war, how did tactics change?

The US Civil War was, as the terms states, a civil was in the United states which started at 1861 and ended at 1865. It started due to the differences between the slave and the free states in terms of power of the government to not allow having slaves in places which were not declared states yet. 

Defensive trench warfare became more common was the tactic change during the civil war.