Final answer:
President Buchanan considered the South's secession illegal but was conflicted over the federal government's authority to coerce states to stay in the Union.
Step-by-step explanation:
President Buchanan declared the South's secession from the Union to be illegal. He recognized that the Constitution stated "the Union shall be perpetual" and believed that the preservation of the Union overrode states' rights. However, he also stated that the federal government had no authority to coerce a sovereign state into remaining in the Union. Throughout the secession crisis, Buchanan struggled to find a solution, holding that although secession was not a legal act, the response to it was equally complex, as he was against the use of force to resolve the matter.