"We say they're Mexican Army, the Mexicans say they're not. Thus if we shoot them, the Mexicans shouldn't throw a fit." Bingo. This was my first thought.
This issue is already long past the stage where a little "international incident" might be in order.
If a WMD is smuggled over the open border instead of just dope and illegals, Presidente Jorge Arbusto El Segundo should be impeached for dereliction of his sworn duty.
U.S. Constitution Article 4 Section 4:"The United States shall guarantee to every State in this Union a Republican Form of Government,
and shall protect each of them against Invasion;"
Invasion: \In*va"sion\, n. [L. invasio: cf. F. invasion. See Invade.] [1913 Webster]
1. The act of invading; the act of encroaching upon the rights or possessions of another; encroachment; trespass.