The fleet had recently completed operations in the Gilbert Islands, taking Tarawa in a bloody and costly invasion in the process. This mission was done for the purpose of obtaining a land base for aircraft to support naval operations against the next objective, the Marshall Islands. The idea that land-based air support was necessary to successfully conduct an amphibious operation was traditional doctrine. The Marshalls would be the first key step in the Navy's march across the Pacific to reach Japan. Mitscher's objective was to weaken Japanese air defenses in the Marshalls and limit their capability of flying in reinforcements, in preparation for a U.S. invasion of the Marshalls, code named Operation Flintlock. Intelligence estimates of the Japanese defenders of the Marshall Islands believed they had approximately 150 aircraft at their disposal. Two days before the intended landings Mitscher's task groups approached to within of the Marshalls and launched their air strikes, fighters first to soften up the defenders, followed by bombers to destroy ground emplacements, buildings, supplies, and the defenders' airfields. It was thought it would take two days to attain air superiority. Though the Japanese battled briskly, they lost control of the skies over the Marshall Islands by noon of the first day. What came next was an aerial bombardment of the Japanese defenses, followed by a naval bombardment from the big guns of Spruance's surface force. The two days of destruction saved a great many lives of the Marines that were landed. The Japanese were estimated to have lost 155 aircraft. Mitscher's task force lost 57 aircraft, from which 31 pilots and 32 crewmen were lost. But the manner in which the fast carrier task force was employed established a pattern for future Pacific operations. In his summary report for the month of January, Admiral Nimitz commented it was "typical of what may be expected in the future."