At the end of June 1942, the Japanese occupied American Territory at Kiska and Attu.
Unacceptable.
The Japanese had assaulted the American Naval base at Dutch Harbor then occupied the two American Islands in the Aleutians. At the same time, they laid a trap for the United States Navy at Midway and the trap backfired. The United States won decisively at Midway, decimated the Japanese fleet, and changed the entire course of the war in the Pacific.
After Pearl Harbor the Empire of Japan raced through the Pacific, attacking and winning pretty much wherever they liked. The United States and her allies reeled in shock. But the American victory at Midway changed all of that. Japan now reeled in shock. The Empire’s days on offense ended forever in June 1942.
If Midway took some of the sting out of the Japanese threat to Alaska and the Aleutians, the threat remained real. More important, Japanese soldiers occupied Kiska and Attu. The United States Army and Navy would correct that unacceptable situation. It would take time. But it would happen.
According to Jerry Coker, in his book First Among Men, three thousand Japanese soldiers occupied Attu and 10,000 occupied Kiska in early 1943. Japan sent a fleet to replenish their supplies, but the American Blockade stopped it in the Battle of the Komandorski Islands on March 26. After that the only resupply would come on submarines.

Because fewer Japanese defended Attu, the Americans chose that island as their first target, loaded a regiment of the 7th Infantry Division on transports in San Francisco in April. Intelligence, vastly underestimating the number of defenders, suggested that the conquest would take at most three days, so they loaded them without arctic cold weather gear. When intelligence revised its estimate sharply upward, they hurriedly loaded the rest of the Division—with tropical gear!
Meanwhile, Japanese commanders ordered defenders on Attu to defend it to the death.
Eleven thousand Americans assaulted Attu in 1943. The Japanese defenders took full advantage of the rugged terrain. They let the Americans land, but every path inland climbed into the mountains and dug in Japanese soldiers had every one of them covered. On half rations at the beginning, the Japanese ran completely out of food and supplies before the end, two and a half weeks later.
Americans in inadequate uniforms fought inland through wind and cold and snow under withering fire. They died in droves. When the fire finally slowed, as the Japanese began to run out of ammunition and began to commit suicide, the mushy thud of hand grenades exploding against human bellies replaced the sound of the guns.

Four thousand of the eleven thousand Americans who fought on Attu died or were injured, one thousand of them from the cold. From the estimated three thousand Japanese defenders, the Americans took twenty-eight prisoners.