Why doesn't the powerful United States unify the North American continent and become the largest country in the world?

Because when the United States was founded, it simply did not have the ability to unify the North American continent. Because North America was colonized by many European countries, such as Britain, France and Spain. Moreover, Canada's territory was colonized by Britain at that time, while Spain's colony was mainly in Mexico, which made it impossible for the United States to expand its territory. Of course, due to the military conflict with Mexico, part of Mexican territory was occupied by the United States, which is now Texas and California in the United States.

Later, due to the change of modern thinking, the United States could not invade other countries' territory at will, because once it invaded other countries' territory at will, it would be strongly condemned by the international community. Of course, the United States will change the current pattern of the North American continent in other ways, that is, actively improve relations with Canada, because the relationship between the United States and Canada is now a military alliance, and the relationship between the two sides is very close.

Although there are some grievances between the United States and Mexico, Mexico's strength is very poor, which also makes Mexico not pose a great threat to the United States at all. However, the problems of illegal immigrants and drug dealers in Mexico have always affected the lives and property of the American people. Now US President Trump has also built a barrier on the US-Mexico border, greatly alleviating the problems of immigrants and drug dealers. If the United States expands to Mexico again, it will also lead to fierce conflicts between the United States and Mexico.

Alaska was bought by the United States from the Russian government. Because the Russian government chose to sell Alaska to the United States in order to solve domestic contradictions, it became the largest overseas territory of the United States.