Which US territory became the first state west of the rocky mountains due to its rabid increase in population during the mid 1800s?

California became the first state west of the Rocky Mountains due to its rapid population increase during the mid-1800s, particularly following the Gold Rush that began in 1848. This influx of settlers and prospectors led to a dramatic rise in its population, prompting California to achieve statehood in 1850. The economic opportunities and promise of wealth attracted thousands, significantly transforming the region.