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.

your answer is

relations between the soviet union and the united states were driven by a complex interplay of ideological, political, and economic factors, which led to shifts between cautious cooperation and often bitter superpower rivalry over the years. the distinct differences in the political systems of the two countries often prevented them from reaching a mutual understanding on key policy issues and even, as in the case of the cuban missile crisis, brought them to the brink of war.

the united states government was initially hostile to the soviet leaders for taking russia out of world war i and was opposed to a state ideologically based on communism. although the united states embarked on a famine relief program in the soviet union in the early 1920s and american businessmen established commercial ties there during the period of the new economic policy (1921–29), the two countries did not establish diplomatic relations until 1933. by that time, the totalitarian nature of joseph stalin's regime presented an insurmountable obstacle to friendly relations with the west. although world war ii brought the two countries into alliance, based on the common aim of defeating nazi germany, the soviet union's aggressive, antidemocratic policy toward eastern europe had created tensions even before the war ended.

hilary clinton

explanation:

trump won the popular vote and ended up becoming the president in 2016 because of this and the electoral college.