The Japanese had invaded the island of Kiska and Attu in 1942, but it was almost a year before the Americans would fight back. When they did, the Japanese felt a very strong American force.
On May 11, 1943, the Americans skipped Kiska and landed on Attu. One fleet landed on Holtz Bay and Massacre Bay. The Americans experienced Japanese defense at it's finest. On top of the Japanese pillboxes, mines, booby traps and other fortifications, Americans had to fight soggy terrain, freezing weather, snow, and the possibility of falling into freezing water.
For the next three weeks, America fought it's way up mountains, through mercky conditions, and through heavily defended areas. However, the two divisions that were on the island finally met and went for the Chichagof Harbor on Attu, which was where the Japanese were based. The Americans finally wiped out the Japanese on June 2, 1943, but not without suffering their own major casualties.
The Japanese weren't done though. There were still troops trapped between Chichagof Harbor and Sarana Bay. The Japanese then threw themselves at the Americans in a suicidal "Banzai" charge. Some Japanese blew their heads off with guns or grenades, others were killed in the push against the Americans. Only about 10 prisoners were taken out of the 2,000 killed by the Americans.
On August 15, 1943, the invasion of Kiska came. However, when American and Canadian troops got there, there wasn't an enemy to fight. The Japanese had evacuated on July 28. How the Japanese escaped American planes, or ships is still uncertain, if not lucky. The Aleutians were taken, and the threat of a west coast attack on Alaska or the American mainland was stopped. It was now the Japanese, who had something to fear from the Americans.
The Marshall islands were next on the American agenda. The Marshall islands were the umbilical cord from Tokyo to the Gilberts. The Americans went for Kwajalein, an island with an area of about 1,200 square miles. The Americans were seeing that "softening" up islands with shells was not enough. American planes just disabled the Japanese airports with aerial bombings.
For the next two months, American forces bombed the island with highly explosive shells that would crater the island. However, when Americans stepped on the island on early February of 1944, they found the Japanese alive, and firing. Americans were in a desperate slow advance.
Pillboxes were America's major hazard, and only dynamite thrown inside the thin slit for firing, or flame throwers could destroy the Japanese inside. However, with American experience, only 356 casualties were suffered by the Americans, while the Japanese lost about 8,120. About 265 Japanese prisoners were taken.
On February 16, The Caroline Islands were a place of war. Admiral Raymond A. Spruance was the leader in this assault on the Carolines. American Air Force planes bombed the island of Truk with everything it had, destroying 200 airplanes, and about 18 ships which consisted of cruisers, ammo ships, cargo ships, oilers, and other ships. Only 17 American aircraft didn't return to their bases.
Truk wasn't charged by the Americans, because the U.S. had neutralized it. In other words, it was a now a worthless island in the Pacific.
Defeating the Marshalls, Carolines, and the Aleutians, the Americans now had the free will to move throughout the western and northern pacific. America was cutting Japan's umbilical cord in the Pacific.