OK, WI the skirmish between the 10th Cav & Mexican federal troops at Carrizal in 1916 led to fullscale war between the US & Mexico ?
US fighting a concurrent war (or another theater of the same one) might prevent us from getting into the war in Europe. Though the Entente or Central Powers might declare one side or the other to be an ally, I don't really expect any practical support. Everyone's too tied up in Europe to send an expeditionary force or anything, though I could expect some naval combat to occur.
I might be in the minority here, but I don't think a US-Mexico war is a complete cakewalk for the States. I have no doubt they would eventually win, but if it turns into guerrilla warfare, then it could go on for a while. Look how much trouble we had with Pancho Villa. Now imagine that on a much wider scale.
And what happens after the war? Do we see a fascist Mexico emerge?