What is the main idea the soviet diplomat is presenting in this passage? the united states is planning to start a war with the soviets. the military power of the united states is starting to decline. the united states is planning on dominating the world economy. the united states does not believe it will have to go to war to dominate the world.

You didn't provide the passage that gives the context of your question.  But I suspect the answer you're looking for is:

The United States is planning on dominating the world economy.

Here's context for that:

Soviet ambassador to the United States, Nikolai Novikov, accused the Americans of imperialism as the essence of their foreign policy, in a telegram he sent sent to the Soviet leadership in September, 1946.  In that telegram, sent about a year after the end of World War II, Ambassador Novikov said:  

Europe has come out of the war with a completely dislocated economy, and the economic devastation that occurred in the course of the war cannot be overcome in a short time. All of the countries of Europe and Asia are experiencing a colossal need for consumer goods, industrial and transportation equipment, etc. Such a situation provides American monopolistic capital with prospects for enormous shipments of goods and the importation of capital into these countries -- a circumstance that would permit it to infiltrate their national economies.   Such a development would mean a serious strengthening of the economic position of the United States in the whole world and would be a stage on the road to world domination by the United States.

Novikov's telegram was much longer than that, but those lines give you a glimpse of his accusations about US economic imperialism.

Corruption i think