Final answer:
President Buchanan's response to secession was to do practically nothing, as he sought to avoid escalating tensions with South Carolina by not resupplying Fort Sumter after an initial failed attempt with the Star of the West.
Step-by-step explanation:
President Buchanan's response to secession was essentially to do practically nothing to directly challenge or thwart the seceding states. Buchanan refused South Carolina's demands to surrender Fort Sumter but made an implied agreement with the state; as long as he did not send supplies, South Carolina would not attack the fort.
However, when Buchanan eventually attempted to send unarmed reinforcements on the merchant ship Star of the West, the militia opened fire and the ship had to retreat. His passive approach maintained a delicate peace until his presidency ended and Lincoln took office.