Buchanan is blamed by many for failing to prevent the Civil War. Before Buchanan left office, all arsenals and forts in the seceding southern states were taken over by those states, except for Fort Sumter, off the coast of Charleston, South Carolina, and three island outposts in Florida. Many of the federal soldiers stationed in Texas surrendered to Texas troops. Knowing that secessionist fervor was strongest in South Carolina, Buchanan made a quiet pact with South Carolina's legislators that he would not reinforce the Charleston garrison in exchange for no interference from the state. But Buchanan never told Charleston commander, Major Robert Anderson, about the agreement, and on December 26 Anderson violated it by moving his command to Fort Sumter. Southerners responded with a demand that Buchanan remove Anderson, while northerners demanded support for the commander.
On January 5, Buchanan sent civilian steamer Star of the West to carry reinforcements and supplies to Fort Sumter. On January 9, 1861, South Carolina state batteries opened fire on the ship, and it returned to New York. Buchanan was again criticized by both sides. Northerners thought that his lack of retaliation against the hostile South Carolina batteries was gutless. Southerners thought him to be a deal breaker for attempting to reinforce Fort Sumter. The lame duck Buchanan made no further moves either to prepare for war or to avert it. On Buchanan's final day as president, March 4, 1861, he said to the incoming Lincoln, "If you are as happy in entering the White House as I shall feel on returning to Wheatland, you are a happy man."
Following his retirement, Buchanan wrote Mr Buchanan's Administration on the Eve of the Rebellion, the first published presidential memoir, in which he tried to defend his actions in preventing the war. I recently read and reviewed that book here. Basically, Buchanan, like so many others of his mindset, believed that slavery was morally wrong, but it was permitted by the constitution and would have eventually died through attrition. The day before his death, Buchanan predicted that "history will vindicate my memory". A century and a half later, this prediction still hasn't come true. Historians continue to criticize Buchanan for his unwillingness or inability to act in the face of secession. Separate polls of historians conducted both in 2006 and 2009 voted his failure to deal with secession as the worst presidential mistake ever made. Historical rankings of United States Presidents consistently place Buchanan among the least successful presidents.
Many years before, when Buchanan was Secretary of State in the administration of President James K. Polk, Polk wrote in his diary: "Mr. Buchanan is an able man, but is in small matter without judgment and sometimes acts like an old maid."