How might grouping several rival ethnic groups into one political unit cause friction once that region gains independence?

By grouping rival ethnic groups, the group can gain independence by fighting together. But after independence, they have differences in their language, their culture, their way of thinking, attitude, perception, communication and many other things which make them incompatible with each other and hence any of these factor(s) can become the root cause of the friction among them.

nuclear fission produces the atomic bomb, a weapon of mass destruction that uses power released by the splitting of atomic nuclei the fighting in iwo jima and okinawa affected the allies' pacific strategy because truman decided to drop the atomic bomb on mainland japan.