John C. Frémont was an American explorer, military officer, and politician, known as 'The Great Pathfinder' for his significant role in mapping the American West during the mid-19th century. He played a pivotal role in the Bear Flag Revolt of 1846, which sought to establish California as an independent republic before it was annexed by the United States. His actions not only impacted California's history but also contributed to the larger narrative of westward expansion and the Mexican-American War.