Why did the USA stop expanding westwards into East Asia? The USA initially started on the east coast of North America, and gradually expanded westwards to reach the Pacific Ocean through a combination of land purchases, wars, and genocides against natives. After that, the USA expanded into the Pacific Ocean (Guam, the Philippines, Hawaii, etc). Why didn't the USA continue its westwards expansion?
In the 19th century, the USA had the opportunity to conquer East Asia by a combination of land purchases, leases, concessions, wars, and genocides against natives of East Asia. This would have been easy to do as Qing China became weaker. The USA could have split China into little states, acquire land, encourage the settlement of China with Americans, and eventually drive the natives onto reservations. This would have incorporated a large and fertile part of East Asia into the USA.
In the 20th century, the USA defeated Japan in World War II and occupied the Japanese Islands. Why did the USA not pursue the same policies that were so successful in integrating western North America and the Pacific into the USA? The USA could have used one of these methods to acquire sovereign land:
Encourage permanent settlement of Americans in Japan, and restrict native settlements (through land purchases, setting up native reservations, restricting internal migration, and genocides). This could be easily accomplished, since Japan was completely defeated, and Americans could exert full control. This could have completely integrated the Japanese islands into the USA, using the same method that was so successful in North America.
Eventually give independence to Japan, but keep some islands. The USA could have kept some strategic islands for itself instead of giving them back to Japan. For example, the residents of Okinawa could have been evicted to the other islands, and Okinawa could be settled with Americans. The strategic Minami-Tori-shima (Marcus Island) could also have been kept for future economic and military purposes. Keeping strategic Pacific islands would reduce the viability of future Japanese military adventures, and forestall any challenge to American power in the region.
Why did the USA not expand into East Asia by using the same expansion tactics it had used so successfully in North America and the Pacific? Surely the USA could have used its superior military technology to conquer native land, used its wealth to acquire more land, encourage American settlement on newly acquired land, and use force to keep the natives within their reservations?