The American continent is geographically inaccessible and secluded, separated by two oceans from any other countries.
This made colonization (or invading a country) of North America which possibly only through a sea. That removed and eradicated nations with large armies as threats. The only power that could influence North America was a nation with a powerful and tough navy, like England’s.
To our north countries like, Canada was resolved by the French, who were our alliances. To the south was Mexico, South America, and Central America.
Thus, at the start, North America had a geographic superiority against any conquering force: distance across an ocean. However, the United States of America can grow and establish their own identities separately from their parent and other countries.
