What is James Buchanan's legacy?

In a nutshell, James Buchanan's legacy was the Civil War. Instead of showing leadership and trying to find a workable solution to the crisis over slavery, Buchanan showed blatant bias toward the Southern states, thus alienating a large swathe of public opinion in the North and adding further fuel to a situation rapidly heading toward civil war.
What the United States desperately needed at that crucial juncture in its history was strong, determined leadership. But for one reason or another, Buchanan was woefully unable to provide it. As a Democrat, Buchanan had secured his election victory of 1856 largely through Southern votes, and so he was reluctant to antagonize the very people who'd sent him to the White House.
So in his time in office, Buchanan supported a number of pro-slavery measures, such as the Lecompton Constitution of Kansas, which greatly antagonized public opinion in the Northern states and exacerbated the already deep divisions between North and South. Buchanan genuinely thought that by supporting such divisive measures, he would hold the Union together. But in actual fact, he was making the eventual split that drove the nation to civil war virtually inevitable.