The answer is the state of California. California is currently the most populated state and the third in the geographical area. It was initially part of the Spanish Empire and name "Alta California". It became the part of Mexico after their successful war for independence. It finally became part of the United States in 1848 after the Mexican American War.
After several claims that gold was discovered within its territory, it was subject to a large level of migration in a period that was known as "The Gold Rush", which lasted between 1847 and 1870. This laid the foundation for the urban development of the state, as it rapidly became a major populated area in the country.