A monopolistically competitive market is, by definition, constituted by a large number of firms that compete producing diferenced versions of a product. Such companies are not price-takers and they hold certain degree of power market and of control over the pricing decisions.
However, in a market that comprises so many actors in its supply side, the market power is splitted in many small units and the amount exercised by each is not very strong. Firms operating in this market structure do not have enough power to affect their rivals through their internal decisions and also not enough power to affect potential competitors and to prevent their entrance. They cannot set entry barriers to prevent the entrance of new companies in the market.
U.S. foreign policy shifted away from "isolationish" to international involvement, mostly during and after World War II, when the United States emerged as the most powerful country on earth.
Good conditions, when everything is plentiful and the people are happy prevent the rise of dictators.
