Final answer:
The expansion of the United States into California and the Pacific region was driven by the concept of manifest destiny and the acquisition of vast territories through the Mexican-American War. The discovery of gold in California attracted settlers from around the world, establishing new relationships with Hawaii, China, and Japan.
Step-by-step explanation:
The expansion of the United States in California and into the Pacific region was caused by the concept of manifest destiny, which believed in the inevitability and righteousness of American expansion. This led to the Mexican-American War, which resulted in the acquisition of vast territories in the Southwest, including California. Subsequently, the discovery of gold in California during the California Gold Rush attracted settlers from various parts of the world, including Hawaii, China, and Japan. This led to the establishment of new relationships with these countries.