Final answer:
Lincoln did not use force at Fort Sumter to avoid enraging Republicans and slave states still in the Union, instead opting to resupply the fort in a non-aggressive manner.
Step-by-step explanation:
The main reason that Abraham Lincoln did not respond with force to the Confederate threat to attack Fort Sumter was that he did not want to anger Republicans and slave states still in the Union. He was initially considering evacuating Fort Sumter, but Northern public opinion and the outrage of his Cabinet at the suggestion of General Winfield Scott to pull out troops shifted his stance.
To avoid any downsides and not incite war, Lincoln opted to resupply the fort in the least aggressive way by sending unarmed ships, which effectively put the decision for war in Davis's hands.