How did the U.S. stop the Japanese aggression in Asia?

The purpose of the Japanese attack on Pearl Harbor in December 1941 was to destroy the American fleet in the Pacific. The Japanese believed that the Americans lacked the will to fight a long-term war and would eventually demand peace.

After this attack, the Japanese persisted in their ruthless conquest of South East Asia, which included forcing the Americans out of the Philippines.

The American fleet in the Pacific was not destroyed by the attack on Pearl Harbor, though; crucially, her aircraft carriers were at sea on that day, which would prove crucial in the ensuing naval engagements.

The US economy swiftly transformed into a war economy, and Japan was unable to match the US's ability to manufacture military hardware.

This did not, however, mean that the defeat of Japan was easy. Because of Japanese militarism and the Bushido code, the Japanese occupying forces met the American advance, island by island, with fierce and even suicidal defense.

The American casualty toll from the seizure of islands like Guam, Iwo Jima, Saipan, and numerous others was extremely high.

The Japanese had amassed a sizable navy, but the days of battleships were over; aircraft carriers and air strikes were crucial to naval victory; the Americas prevailed at battles like Midway and Leyte Gulf, while super battleships like the Yamato proved to be ineffectual. Nevertheless, the Americans were winning.

However, the enormous casualty toll from capturing the islands made the Americans think that an invasion of mainland Japan would result in an intolerable number of American deaths; hence, the decision was made to unleash the atomic bomb, first hitting Hiroshima and then Nagasaki, until the Japanese surrendered.