Who had the mission of invading south-west and California in 1840s?

In the 1840s, the United States was engaged in the Mexican-American War. The war began in 1846, when the United States annexed the Republic of Texas, which Mexico considered to be its territory. In response, Mexico declared war on the United States.

One of the key figures in the Mexican-American War was John C. Fremont. Fremont was a young Army officer who had been exploring the American West. In 1846, he was given the mission of invading south-west and California. Fremont's forces were successful in capturing California, and he became the first military governor of the territory.