Midway proved to be the last great naval battle for two years. The United States used the ensuing period to turn its vast industrial potential into increased numbers of ships, planes, and trained aircrew. At the same time, Japan, lacking an adequate industrial base or technological strategy, a good aircrew training program, or adequate naval resources and commerce defense, fell further and further behind. In strategic terms the Allies began a long movement across the Pacific, seizing one island base after another. Not every Japanese stronghold had to be captured; some, like Truk, Rabaul, and Formosa, were neutralized by air attack and bypassed. The goal was to get close to Japan itself, then launch massive strategic air attacks, improve the submarine blockade, and finally (only if necessary) execute an invasion.

Answer this question, if possible (if impossible, reply "unanswerable"): How were the bypassed Japanese strongholds neutralized?
air attack