Buchanan isn't bad because he did things; he's considered an awful president because of his general lack of action. For example, he had the opportunity to strengthen Federal installations in the South, provide more men, etc. There was a significant military draw down under Buchanan, see The Regular Army Before the Civil War: 1845-1860.

One of the more incompetent episodes involves the failure to finish Fort Sumter in time, despite the fact that construction had started in 1829. It was already well known that Fort Moultrie (where Anderson's troops originally were) was almost indefensible. Yet, construction was slow in the best of times and basically stagnant most of the time. Had the fort actually been finished, it's conceivable that Anderson would've been able to hold out much longer than he was. A History of Fort Sumter: Building a Civil War Landmark is probably the best book here.

Beyond the general incompetence of Buchanan's administration, Buchanan's Secretary of War, John Floyd, was sympathetic to the Confederate cause, and actually sent shipments to the South to be captured when the Confederacy inexorably seceded. See John B. Floyd's The Militant South: 1800-1860.

Now, I realize that my answer is largely reserved to military matters, which is a function of what I know about Buchanan's policies/actions (or lack thereof).