Simplify the following text for a Grade 9 native English-speaking person.
Original: The XX Bomber Command operating Strategic B-29 bombers of the Twentieth Air Force , which were attacking Japan in Operation Matterhorn , were forced to move as well , but although this affected their efficiency for a short time , in early 1945 the Twentieth Air Force moved to newly established bases in the Marianas under the command of the newly established XXI Bomber Command .
Simplified: But this affected their efficiency for only a short time . In early 1945 the Twentieth Air Force moved to newly established bases in the Marianas under the command of the newly established XXI Bomber Command .