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.
I assume from your question that in this exercise you are imagining yourself to be the US President or his advisors.
Responses in a case like this would be military, using naval and air forces to support Mexican military operations, perhaps including transport of US troops to Mexico to assist. Also, bombing strategic military stations in Cuba would weaken the Cuban military's ability to support its invasion.