Citadel cadets fired on the re-supply ship ‘Star of the West’ in January 1861 while Buchanan was still President.
Buchanan opposed secession but he also believed that the Constitution didn’t grant Presidents the right to force States to remain in the union.
His view on force was entirely different when it came to Territories. In 1857 Buchanan sent the army to put down a Mormon rebellion against the Territorial Government.
The Star of the West invaded South Carolina’s territorial waters. They fired to drive it away.