I know this is just some hypothetical fantasy, but where it falls down is the idea that Central and South America would declare war to "evict the colonists" from North America.
First, they'd have to evict themselves first. Mexico (i.e., Central America), as well the rest of Central America and all of South America is comprised of people of European descent. With all the nations in that area other than Brazil, they are of Spanish and indigenous descent, whereas Brazil is Portuguese with some indigenous mixture.
Any war that would have the CCP as a partner would mean that China would assert dominance over the US west and oppose any removal of people of Asian descent and turn on the Central and South Americans and probably enlist the aid of Russia as they are already allies. In the end, Central and South America would not only not have gained any territory, but be fighting for their own lives.
On to the hypothesis of the US using nukes at any time. Simple, only if an actual war broke out and we are attacked. So, the military would send up nukes only if they saw nukes coming in. Mutual destruction.
There is an exception. If a nuke was smuggled into the US and detonated AND the US could in some way determine the Uranium radiation signature's origin to say, Iran, China, North Korea, Pakistan, or Russia, then I could see the government giving the green light to sending nukes to whoever they determined was the culprit.