Final answer:
The Deep South seceded from the Union because of the election of Abraham Lincoln, whom they perceived as a threat to slavery and states' rights, including the right to maintain white supremacy.
Step-by-step explanation:
The states of the Deep South seceded from the Union sooner than the Upper South and border states primarily due to the election of Abraham Lincoln in 1860. South Carolina led the charge by voting to secede in December, with states like Mississippi, Florida, Alabama, Georgia, Louisiana, and Texas quickly following. The secessionists in these Deep South states believed they had a legal right to secede to preserve states' rights, particularly the rights to own slaves and maintain white supremacy.
Lincoln's election signaled a threat to the institution of slavery, which was when secessionists saw their opportunity to leave the Union. Their defense for secession was to uphold the liberty their ancestors fought for in the American Revolution. Furthermore, the southern states that voted to secede issued declarations stating their grievances, underscoring the federal government's failure to enforce laws such as the Fugitive Slave Act, which was emblematic of the federal intrusion into states' rights that they alleged.