The Great Depression was a period of economic and social upheaval in the United States of America in the 1930s. Many people became unemployed and homeless; the period was described as quite barbaric by Spock.
The misery of the Great Depression was the backdrop for James T. Kirk's visit to 1930. An overdose of cordrazine had rendered Leonard McCoy temporarily insane, and McCoy had fled through the Guardian of Forever – altering the past in a way that eliminated the Federation from the timeline. Pursuing him, Kirk and Spock also journeyed to Earth's past, there encountering social worker Edith Keeler. Through Keeler, they were eventually reunited with McCoy. Kirk also fell in love with the young woman – tragically, since she was fated to die. (TOS: "The City on the Edge of Forever")