If nothing else, taken a moral stand; he should have reinforced the forts; he should have begun moving troops to the border to show this was not a game.
Most important, he should have begun these measures in 1857.
BTW, Buchanan supported the “English Act” that tried to bribe Kansans into accepting slavery with more land. Disgusting.
If he had made any moves that appeared to “threaten” the South or the institution of slavery at any point, that would’ve given the South the excuse to move on secession. If Fremont had won instead, that would’ve triggered it 4 years ahead of time. Well, nothing you don’t already know. ;-)
The English Bill could simply be viewed as a partisan action to make KS a Democrat state. It wouldn’t make much political sense for Buchanan to have championed the creation of a Republican state.