 Complete the following phrase:  Upon completion of the Marshall invasions, United States forces had pushed to a point about midway between Pearl Harbor and Japan. By June, invasion forces were assembled the attack on the Mariana Islands, the next island group on the way to the Japanese homeland. On 15 June the Marines landed on Saipan and three weeks later the island was considered secure for American forces. Within another week, the Seventh Air Force moved the 48th Bombardment Squadron to Saipan to assist in softening up enemy defenses preparatory to the next invasions in the Marianas. For a week the 48th Squadron and other units bombarded Guam and Tinian. Then on 21 July, United States forces invaded Guam and three days later 