Sometime simple acts and events send ripples through history. For want of a nail... One small example: A small air raid ends an empire.
We are about a week away from the 75th anniversary of the Doolittle Raid. A few months after the Japanese attack on Pearl Harbor brought about US entry into WWII, the US retaliated with an air raid on Tokyo. Sixteen specially-modified B-25 Mitchell bombers took off from the deck of U.S.S. Hornet and bombed Tokyo, then escaped to the Asian mainland. Nearly all the planes were lost in Japanese-occupied territory, and several of the aircrews as well. They inflicted minor damage, destroyed a few important buildings, damaged a Japanese aircraft carrier that was nearing completion, and killed a few hundred people. As a morale-boost, striking back for Pearl Harbor, it was valuable, but the damage inflicted was almost insignificant.
The indirect effects, however, were far more significant. During the attack, the Japanese military made widespread use of coded transmissions in an attempt to coordinate the response to the attack. Japanese leadership decided to rearrange their defenses, and wanted control of Midway Island. The bounty of transmissions the Japanese produced allowed US cryptographers to decode Japanese naval signals and ambush the Japanese carrier fleet. The Battle of Midway was a major victory for the US and a turning point in the war in the Pacific.