Submarines were important in World War I because England relied so much on supplies brought in from abroad. England, which was one of the main powers on the Allied side, relied on supplies from the United States and from places like India that were in its empire. The Germans wanted to block those supplies and choke off the English economy. They did not have a strong enough surface navy and so they relied on submarines. This made submarine warfare (and tactics that were used against submarines) extremely important in World War I.