(covers information from several alternate timelines)
On Earth, the United States experiences the Great Depression. In Europe, Adolf Hitler's Nazi movement establishes a dictatorship, and eventually World War II breaks out. In an alternate timeline, activist Edith Keeler gains enough political influence during this decade, delaying the American involvement in the War.
The films Frankenstein, Bride of Frankenstein, Son of Frankenstein, A Night at the Opera, The Wizard of Oz, The Black Cat, Death Takes a Holiday, and I'm No Angel are released in this decade, as well as the songs "Goodnight Sweetheart", "You Are My Sunshine", "It's Only a Paper Moon", "Isn't It Romantic?", "I've Got the World on a String", "I've Got You Under My Skin", "If I Only Had a Heart", "The Way You Look Tonight", "They Can't Take That Away from Me", and "I'll Be Seeing You". Clark Gable is a major movie star of the era.
- Civil war breaks out on the planet Xindus. (ENT: "The Shipment")
- In an alternate timeline, Edith Keeler is saved from the traffic accident by Leonard McCoy, allowing her to continue striving towards her goal of world peace by founding one of the largest peace movements in the United States of America. By the late 1930s, the growing pacifism caused by this movement and others caused the United States to focus on diplomacy and ultimately delay its involvement in World War II. (TOS: "The City on the Edge of Forever")