One important reason many slave states seceded from the Union was? 1. the election of Jefferson Davis
2. the election of Abraham Lincoln.
3. the attack on Fort Sumter.
4. The attack on Fort Wagner.

The main reason the us senate rejected the treaty of versailles was because, isolationists felt the us should not have gotten into the war. republicans had gained power in the senate. democrats had gained power in the senate. european nations wanted to punish germany.