Final answer:
The upper southern states seceded from the Union after President Lincoln called for 75,000 volunteers following the Confederate attack on Fort Sumter, which signaled the beginning of the Civil War.
Step-by-step explanation:
The upper southern states seceded from the Union for a culmination of reasons related to the tensions between the North and the South, especially over slavery and states' rights. However, it was President Abraham Lincoln's call for 75,000 volunteers after the fall of Fort Sumter that spurred the upper southern states to make their final decision on secession. This call to arms was issued on April 15, 1861, after Confederate forces attacked Fort Sumter, signaling the start of the Civil War.
The decision by states like Virginia, North Carolina, Arkansas, and Tennessee to secede was deeply tied to the perception that the federal government was mobilizing to suppress what they saw as their rights, including the right of self-determination and the institution of slavery.
For these states, Lincoln's actions, perceived as an aggressive move against the South, caused them to join the Confederacy. Contrariwise, states such as Delaware, Maryland, Missouri, and Kentucky chose to remain with the Union. The complexities of the secession crisis and the beginning of the Civil War highlight the deep divisions and conflicting ideologies that were present in the United States at the time.