The United Nations has historically been the place to ask for aid and sometimes for mediation when one nation attacks another - as in Korea in 1950 or Kuwait in 1990.
Most nations in North and South America are members of the Organization of American States the OAS, which is, among other things, a mutual defense treaty to support one another in the event of attack.
The reason why the British wanted to conquer this area was because the Southern colonies were incredibly important trading ports, which meant that if the British controlled them they could cut off lots of revenue to the states.
