What was the confederacy's strongest resource in the Civil War?

This question requires a subjective, opinionated answer. With that said, many historians will suggest that the strongest resource for the South at the very beginning of the war was its cotton crops. Even with a blockade by the Union, cotton from the South was able to reach European markets. In fact, for most of the war, a permit from the Federal government allowed Northern companies to buy cotton from the South. Also, it may be suggested that the South's greatest resource at the beginning was a strategic one. This was due to the large amount of territory of the South and its huge coastline. In addition to that was the fact that the South would only need to fight a defensive war for its victory. On the other hand, the North had to invade and fight on unfamiliar soil and destroy all Confederate resistance. It's clear that there are other valid opinions.