Also not really about the Monroe Doctrine, as the US action was in response stri...

Also not really about the Monroe Doctrine, as the US action was in response strictly to the deployment of offensive missiles (what it saw as a de facto aggressive action against it) -- not the simple fact of Cuba forming a close relationship with the USSR per se.

Also, if government of Mexico ever decides to invite the People's Liberation Army into Mexico to help Mexico defend itself

And if that invitation happened only after Mexico was invaded by the US (on as grounds as equally stupid and unprovoked as Russia's current invasion of Ukraine) -- then I'm sure we can trust that you will not only unequivocally condemn that aggression, but solidly champion Mexico's right to defend itself against it by whatever means necessary.