Why didn't the United States unify America and become the world's largest territorial power?

The United States really wants to rule the whole America and become the largest country in America. However, there are many factors that prevent the United States from ruling the whole America and establishing a unified nation-state for the following reasons.

First, the lessons of ethnic contradictions. The United States is the largest immigrant country in the world, with more than 50% of its domestic population coming from all over the world and few native Indians. Judging from the American Civil War, the United States paid a huge price for racial equality and faith. At present, racism and politics in the United States have always been a big problem, which not only brings great pressure to public security and management in the United States, but also has a great impact on the national development prospects of the whole United States. Different beliefs of different ethnic groups lead to different demands and aspirations of the people of that country. If the United States is unified, it will be a huge project, which will not only integrate Mexico and Canada, but also bring a fatal crisis to the American economy, and may also lead to the division of national beliefs in the whole country, so that the multi-ethnic federal country that the United States has finally established will fall into chaos again. Secondly, the international environment does not allow it. However, the United States during World War II is still the United States today. It is extremely difficult to unify the whole America, because there are competing Soviet Union or Russia, and there was once a European unification. Every country in the world has paid a great sacrifice and price for its political power and national independence. At this time, if the United States wants to annex other regions into its own territory, it will be bravely resisted by other countries. Constant condemnation will prevent the United States from going further and further in the confrontation between world powers, which is extremely uneconomical for the United States. In addition, the United States has established the American trade system, which can be regarded as the economic lifeline control of the Americas.

Finally, the United States has another way to control the whole United States. Today's United States basically relies on the help of the United States and the trading system. It can be said that the United States can achieve another kind of ideological control by constantly controlling the economic and cultural infiltration of other countries in the Americas.

If there are any omissions, please add them.