What were the strengths of the South in the US Civil War?

they had experienced officer corps

and the south was closer to the supplies they needed, plus they were fighting on their own territoriy
Some of the best officers of the US Army had resigned to join the Confederates.

The Confederate rank-and-file were the sort of men who actively relished fighting.

Stronger cavalry tradition - most young men owned at least one horse.

Most of the battles were on ground unfamiliar to the enemy

More emotive mission - to defend the homeland against the invader