I noticed you didn't answer. Would you support the US agreeing to a peace deal with Mexico if they invaded that gave them CA/AZ/NM to save lives? A simple yes or no will suffice.
And they are negotiating, do you mean that he refuses to roll over and give Russia what it wants?
If Mexico had the capability to exterminate the entire US, and nobody was coming to help us, then yes.
Consider it a lesson on military spending, training, and equipment that should be rectified ASAP.
P.S. This isn't a videogame, there's nothing preventing a counter offensive at a later date when you're better prepared to retake territory lost. Play the long game and play it smart.