Air bases refueled, rearmed, and repaired military air units. The first combat planes had limited striking range, thus their effectiveness relied on air bases near the front lines. In WWI, military engineers continually constructed air bases as the front moved further from previous air bases. Air bases again played a crucial role in WWII as the Americans advanced toward Japan. Aircraft carriers could not transport large bombers, so Americans constructed crude air bases on captured islands in order to increase the effectiveness of its bombing campaign. After WWII, mid-air refueling stretched the striking distance of bombers, though they still relied on air bases to rearm and repair between sorties.