The United States government doesn't meddle much into economy. This is because it's based on liberal laissez faire principles which state that the government should not impose it's own business practices on the economy. You can see that by the fact that anyone with money can open up their shop or food place or anything similar. You can also see that in a new IPhone that is being released every year for hundreds of dollars and everyone buys it. There is a high chance that in a government controlled economy that would not be a possibility because the government would control investments and production. What the government does do is prevents things like business malpractices, fraudulent behavior, monopoly establishment, and anything other that could harm your right to participate in the economy and to open up your own business.
As settlers started moving west in the 1800s, counties, states and other land was drawn up in a grid-shape so it was easier to distinguish who owned a certain property.
The political and economic characteristics that made possible United States expansion through the different events in the past, are very different from today. It could be possible for the United States to expand in the present, but certainly, it would happen by negotiations and following the international organizations pacific means.
