John Charles 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 promote westward expansion. Frémont's explorations contributed to the knowledge of areas such as the Oregon Trail and California. In addition to his exploration work, Frémont was a key figure in the early Republican Party and ran for president in 1856 as the first Republican candidate. His political career was marked by his strong opposition to the expansion of slavery, making him an important figure in the lead-up to the American Civil War.