Which confederate general was previously asked to lead the union army?

Robert E. Lee

He disapproved of secession, and asked the General-in-Chief of the Union Armies whether he could stay neutral - to which he got a scathing reply. ("We don't want any fence-sitters!")

When Virginia seceded, he declared that he could not draw his sWord against his beloved home-state, and resigned from the US Army to join the Confederates.