What Constitutes an "Invasion"?
The Founders had invasion in mind. They mentioned it four times in the Constitution, without ever defining it. Here are the citations:

Article I, Section 8, Paragraph 15: The Congress shall have the Power (to) provide for calling forth the Militia to execute the Laws of the Union, suppress Insurrections and repel Invasions;

Article I, Section 9, Paragraph 2: The Privilege of the Writ of Habeas Corpus shall not be suspended, unless when in cases of Rebellion or Invasion the public Safety may require it.

Article I, Section 10, Paragraph 3: No State shall, without the Consent of Congress, lay any Duty of Tonnage, keep Troops, or Ships of War in time of Peace, enter into any agreement or Compact with another State, or with a foreign Power, or engage in War, unless actually invaded, or in such imminent Danger as will not admit of delay.

Article IV, Section 4, Paragraph 1: The United States shall guarantee to every State in this Union a Republican Form of Government, and shall protect each of them against Invasion; and on Application of the Legislature, or of the Executive (when the Legislature cannot be convened) against domestics Violence.

Brutal Mexican drug gang crosses into U.S.
Zetas butcher victims to spread message of fear
The signature crimes of the most violent drug cartel in Mexico are its beheading and dismemberment of rival gang members, military personnel, law enforcement officers and public officials, and the random kidnappings and killings of civilians who get caught in its butchery and bloodletting.

But this disparate band of criminals known as Los Zetas is no longer just a concern in Mexico. It has expanded its deadly operations across the southwestern border, establishing footholds and alliances in states from New York to California. Just last year, federal agents tied a cocaine operation in Baltimore to the Zetas.
http://www.washingtontimes.com/news/2011/apr/19/violent-mexican-drug-gang-expands-into-us/