Let's imagine a situation similar to Israel's where the US is the one under attack. This is actually possible given the right chain of events.
First, let's assume that Andres Manuel Lopez Obrado follows through with his threat to start a civil war in Mexico unless he is made president. Mexico has had some provinces, mainly in the south, rebelling for years so this is quite possible. Now, let's assume that Venezuela's President Hugo Chavez takes advantage of leftist anti-Americanism and starts providing support for the rebels, both in cash and in weaponry. Given civil conflict in Mexico, this is likely.
This next is a bit of a stretch but let's assume that the Mexican rebels decide that they want part of the US back. La Raza has demanded Texas, Arizona, New Mexico, and California so it could happen. In order to force the issue, they set up near the border and start lobbing missiles into the US. Most of these are un-aimed short-range missiles and mainly threaten cities within a hundred miles of the border but they also show off some long-range missile's that are part of Chavez's billion dollar purchase of arms from Russia. These can hit any city in the US. Further, Chavez announces that he is pursuing nuclear and biological weapons for self-defense.
So what would we do? Just sit and take it, knowing that worse was coming? Invade Mexico knowing that there would be civilian casualties? Attack Venezuela directly knowing that it would likely turn all of South America against us? Would we get any sympathy from Europe or the UN?
There is no good solution but we would at least be in the position of being strong enough and large enough that we would not be in danger of annihilation.