That's not exactly true. When Pancho Villa attacked the United States, the United States sent troops to Mexico to kill or capture him. Likewise, when Osama Bin Laden attacked the United States, we sent troops to Afghanistan to kill or capture him. etc.
The idea that States war against States, exclusively, is extremely outdated (circa 1800 AD...) and may never have been accurate in the first place.
I'll give you that for sure, especially since Mexico is pretty much in a battle for control of the country right now. Seems like Mexico's war on drugs could easily slip into some sort of bloody coup if the Mexican government "loses."