People fight for different reasons. But it is true that the secession movement in the Deep South was motivated by a desire to preserve slavery. One doesn't have to believe that every Confederate soldier -- or even most CSA troops -- fought for that reason, to recognize what drove secession and led to war.
Slavery was built into the constitution. The slave states - New Hampshire, Massachusetts, Connecticut, New York, New Jersey, Pennsylvania, Delaware, and Maryland made sure of that. Oh, and the states of Virginia, North Carolina, South Carolina, and Georgia too.
You might say that in 1861 the Deep South was motivated by a desire to preserve the Constitution.