All the following were strengths of the confederacy except
the ability to wage a defensive war
shorter supply lines
the resources of the upper south states
a strong navy

I'd say the first one. The ability to wage a defensive war.

china unilaterally claims the entire south china sea as sovereign territory, then builds artificial islands to bolster its claim. it suddenly activates an air defense identification zone over the east china sea, and threatens to down any aircraft that does not report its position. it builds roads into indian territory, then redraws the maps to show that it is actually chinese territory. the people's republic under president xi jinping is quickly becoming the bully of asia.