Final answer:
The southern states initially lacked the military strength to take Fort Sumter, which became the flashpoint leading to the American Civil War when Confederate forces bombarded the fort in response to a resupply attempt by Lincoln.
Step-by-step explanation:
The Union military installation that the southern states initially did not have sufficient military power to take was Fort Sumter. During the secession, Confederate forces had already begun taking over federal forts, with Fort Sumter being a strategic point due to its location in Charleston, South Carolina, an important Atlantic port. The fort was under threat because the small Union garrison was unable to secure supplies; the local merchants refused to sell food to them, leading to a dire situation. President Lincoln faced a tough decision on how to respond without triggering a war. However, when Lincoln attempted to resupply the fort, the Confederates interpreted it as an act of aggression and on April 12, 1861, commenced a bombardment on Fort Sumter, leading to its eventual surrender and marking the start of the American Civil War.