What american strength did yamamoto hope to weaken in attacking midway?

Due to their advanced code-breaking abilities, the United States was able to predict what the Japanese's attack strategy on Pearl Harbor. The U.S. strategically moved their aircraft carriers to avoid the attack and were able to advance on the Japanese Navy. 

The aircraft carriers then became the focus of Japan's military strategy. They were upset that their plans were interrupted and didn't work the first time, so they gave it another try. The Japanese fleet commander, Admiral Yamamoto, chose Midway since it was close to Pearl Harbor. His goal was to destroy the American aircraft carriers. He hoped that this tactic would move them away from the rest of the U.S. forces and be the best place to eliminate the carriers.