The Monroe Doctrine for example has never included a drive to conquer Latin America and annex it. That doctrine is about driving out European colonialism. The territorial integrity of countries in Latin America has been well served by that shield. If you're Venezuela and your entire nation has collapsed (which it has), you don't have to fear someone conquering you due to extreme weakness and annexing you: the US would never allow that and everyone knows it. Historically, at nearly any other time or place, Venezuela would get invaded and conquered while they're so extraordinarily weak (they have the largest oil reserves on earth after all).

If the US were to behave like China is already (much less what might come next), the US would be threatening to annex Toronto and Vancouver, along with Baja Mexico and the entire Gulf of Mexico and Caribbean Sea.

We don't have to wait for bad behavior out of China. They just used their military to steal territory about four times the size of Texas from their neighbors.