Honestly Buchanan wasn’t a great president, but I don’t think it really would have mattered what actions Buchanan could have taken.

By the time he got into office civil war had all but been guaranteed as decades of kicking the can on the issue of slavery, combined with increasing resentment in the divide of the north and south culturally. Had created a powder keg that arguably was lit back during the start of bleeding Kansas years before he even became president.

And with how stubborn the south was on not letting go of slavery, well there really was no diplomatic solution by then. Hence kicking of the can all those years prior. Maybe he could have enacted decisions that delayed the war, but it would have only been a delay.

The only way civil war could have ever been prevented is if the founding fathers had told southerns to pound sand and outlaw slavery back during the signing of the constitution. That or maybe if the cotton gin had been invented a decade later