That’s not history but ok
Answer:
In order to keep the Union together, Buchanan could have taken leadership initiative and anticipated what the secessionists had planned after the election. He could have made efforts to deescalate.
He could have made better utilization of Secure Federal arsenals and assets. Maybe even reassign loyalist military commanders to forts in the South.
The military tactic that Secretary McNamara and General Westmoreland advised to President Nixon in order to win their battle in Vietnam was they proposed to the president that the territories occupied by Viet Cong should be continuously bombed to continue the advancement of American forces.