The immediate cause was because President Carter decided to allow Iran's desposed Shah into America so he could receive cancer treatment.
Before Korean war
The Vietnam were divided in two nations in 1954 to 1975 and the Korean war was 1950 to 1953
Referring to their role as major players of trade routes in Asia and the indian ocean.
They managed to fulfill this roles due to their superior exploration technology and weapons. The European developed the sea exploration technology at least one century befor the East Asian countries and around 400 years before the South Asian