John C. Frémont was an American explorer, military officer, and politician in the 19th century. He is best known for his expeditions in the western United States, which helped map the region and opened it up for settlement. His adventures earned him the nickname "The Great Pathfinder," and he played a significant role in the California Gold Rush by leading settlers to the West. In addition to his exploration work, Frémont was also a key figure in the early Republican Party. He ran for president in 1856 as the first Republican candidate, advocating against the expansion of slavery into new territories. His efforts made him a prominent figure in American history.