Final answer:
The Civil War began when Confederate forces fired upon Fort Sumter on April 12, 1861. This event marked the start of the conflict after escalating tensions over slavery and state rights.
Step-by-step explanation:
The Civil War officially started when Confederate forces fired upon Fort Sumter on April 12, 1861. This aggressive act took place in Charleston's Harbor, South Carolina, and marked the outbreak of one of the bloodiest conflicts in American history. Before this, tensions were high due to the secession of Southern states following the election of President Abraham Lincoln, whose platform opposed the expansion of slavery.
The attack on Fort Sumter galvanized the North and led President Lincoln to call for seventy-five thousand volunteers to suppress the Southern rebellion, underlining the Union's determination not to let secession proceed. On the other hand, the South was dedicated to defending its new Confederate nation and maintaining its institution of slavery.