In OTL, the Southern sector of the District of Columbia was returned to the state of Virginia in 1847. What if it was had been still a part of the F4ederal capital in 1861? Would this have had any affect on Virginian secession and the coming war?
Probably not on the Virginian Secession, but if DC's defences extend to the south bank of the Potomac and are in Union hands at the start, that gives the Union a potential staging point and the Confederacy an extra target to capture.