Question

What was the Monroe doctrine?

Answer 1

The Monroe Doctrine was a US foreign policy regarding Latin American countries in 1823.

Answer 2

The Monroe Doctrine basically was like this.

America: Stay away from North America you darn colonizing Europeans!
Europe: Oh yeah? What're you going to do then?
America: We're going to beat you up really badly!
Europe: ...Fine.

For lack of will to properly explain it that's the one in a nutshell.