Marching toward the Guam invasion, July 15, 1944
'Softening' was the military euphemism this day. Another way to put it would be the reduction of Agana and the captive U.S. Naval Station to the south to rubble. Ship borne artillery was also pounding Rota, along with bombing raids by carrier-based planes.
As this United Press story put it, "Ammunition dumps, storage facilities and gun positions were damaged severely on both islands by American bombers, which carried out the attacks without loss."
Meanwhile, the big Pacific war story being seen on the mainland was the execution of captured B-29 pilots by the Japanese, making the case that the airfields on Saipan and Guam would have to be taken to successfully prosecute the war against Japan with long range bombers.
A pretty hyped up, 2014 History Channel video is here, with characteristic abuse of the archival documentary film provided by the U.S. Navy. It may surprise Chamorros on Guam to learn that they are 'Chamorro Indians.' You might have expected that from a 1944 production.