Mexican soldiers on Arizona soil held a U.S. Border Patrol agent at gunpoint Sunday night. The Mexicans retreated after backup agents responded.

This is far from the first time the Mexican military, and/or those wearing Mexican uniforms, most likely members of Los Zetas, have crossed over the border, in support of drug and illegal immigration operations. Though both governments have sought to downplay such incidents, the increasing number of these sometimes violent incursions – over 200 confirmed incursions since 1996 - makes the situation difficult to sweep under the rug.

From The Washington Times: Agents assigned to the Border Patrol station at Ajo, Ariz., said the Mexican soldiers crossed the international border in an isolated area about 100 miles southwest of Tucson and pointed rifles at the agent, who was not identified.

It was unclear what the soldiers were doing in the United States, but U.S. law enforcement authorities have long said that current and former Mexican military personnel have been hired to protect drug and migrant smugglers.

"Unfortunately, this sort of behavior by Mexican military personnel has been going on for years," union Local 2544 of the National Border Patrol Council (NBPC) said on its Web page. "They are never held accountable, and the United States government will undoubtedly brush this off as another case of 'Oh well, they didn't know they were in the United States.'

"It is fortunate that this incident didn't end in a very ugly gunfight," saidthe local's posting.

Not for the first time, U.S. officials kept mum on the subject. The Border Patrol spokesman did not respond to the newspaper’s questions, and a State Department spokeswoman claimed her department had no information, and referred questions to the same Border Patrol that refused to return the Times’ phone calls.

The Mexicans issued a statement, which, as such statements often are, could beviewed as blandly self-serving: Ricardo Alday, spokesman at the Mexican Embassy in Washington, said Tuesday that Mexico and the United States are engaged in "an all-out struggle to deter criminal organizations from operating on both sides of our common border."

"Law enforcement operations have led, from time to time, to innocent incursions by both U.S. and Mexican law enforcement personnel and military units into the territory of both nations, and in particular along non-demarcated areas of our border," he said.

"We always try to solve these incidents in a cooperative fashion, and as acknowledged by the Border Patrol, this was the case in the episode at Ajo," he said.

Other incidents have proved even more dramatic. From the paper:

A year ago, U.S. law enforcement authorities were confronted by gunfire from automatic weapons as they chased and caught a drug-smuggling suspect in Texas trying to flee back into Mexico, the Hudspeth County (Texas) Sheriff's Office said.

No one was hurt in that incident, and the gunmen were not identified, although the area has been the scene of similar incidents over several months, including a confrontation in January 2007, when heavily armed men in Mexican military uniforms fired on Texas officers with a .50-caliber machine gun mounted on a camouflaged Humvee.

Themen were identified at the time by Hudspeth County Sheriff Arvin West as "soldiers."

Inthat incident, Hudspeth County deputies pursued three sport utility vehicles back to Mexico after spotting them driving north from the Rio Grande. The pursuit ended on the U.S. side of the border when the deputies encountered 10 heavily armed men in what they described as battle-dress uniforms.

At that time, deputies found 1,400 pounds of marijuana in one of the vehicles abandoned after it blew a tire early in the pursuit. Another made it into Mexico and a third got stuck in the Rio Grande and was burned by the "soldiers" after it was unloaded.

In November 2007, the Border Patrol chased a dump truck full of marijuana in the same area when it also got stuck in the river while trying to return to Mexico. While agents sought to unload 3 tons of marijuana, the driver - who had fled - returned with a heavily armed group of men wearing Mexican military uniforms and carrying military-style weapons.

The soldiers backed the agents away and bulldozed the truck back into Mexico.

No suspects were ever identified and arrested.

A coalition of Texas border sheriffs has demanded that both the U.S. and the Mexican governments do something before the situation spins out of control.

Sheriff Sigifredo Gonzalez Jr. of Zapata County, Texas, who founded the coalition, said a growing number of suspected incursions and violence aimed at the area's law enforcement officers is making the border "a pretty dangerous place."

Sheriff Gonzalez said three of his deputies in 2006 spotted 25 men dressed in military uniforms in the U.S. during a late-night patrol. He said the men marched two abreast and carried duffel bags and automatic weapons, and that his "outmanned and outgunned deputies" were forced to retreat.

"The only thing you can do in that kind of situation is seek cover," Sheriff Gonzalez said. "I'm not going to lose someone in an unfair fight."

Speaking anecdotally, rogue ex-military units like Los Zetas generally do not enforce military marching discipline. The truth is that the invaders are both military and criminal gangs. To be more specific, it is no secret that Los Zetas control and manipulate the border, and it is also no secret that elements of the Mexican Army, as corrupted as much of the rest of the Mexican government, are involved in drug running and human smuggling.

From Texas to Arizona to California, we are watching as the border disintegrates under the pressure of a collapsing and corrupt Mexican state, and a distant and feeble American response.