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:  

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

"a government in which power is divided between state and national levels". federalism is a system of government in which entities such and state and province, share power with the national government.