How important was the concept of the frontier to america's political and diplomatic development?

It depends what time period, but during ww2 or ww1 really, America's people were afraid of communism and groups like the America first committee and other groups were created to keep america isolationist. We did not want to intervene with war, but it was inevitable because we had trade relationships, which in both wars, were at risk. 

Belgium

Explanation: Although Leopold II established Belgium as a colonial power in Africa, he is best known for the widespread atrocities that were carried out under his rule, as a result of which as many as 10 million people died in the Congo Free State.