Dr. McCoy accidentally injected himself with cordrazine. It was 100 times the normal amount. Dr. McCoy went berserk and fled the bridge. He went to the transporter room and beamed himself down to the planet. Dr. McCoy jumped through the Guardian of Forever and changed history by saving Edith Keeler from being killed in a traffic accident. Edith Keeler started a pacifist movement which delayed the United States' entry into World War II. This gave Nazi Germany time to develop the atomic bomb first, win the war, and take over the world. The Guardian of Forever said "Your vessel, your beginning, all that you knew, is gone."