Something like a full scale drug war is going on in Mexican cities just south of the United States. Mexican drug gangs roam major cities at will, in daylight, killing or kidnapping anyone that gets in their way. The local police look the other way, having either been intimidated, or "bought." Whole cities are managed for the primary purpose of pushing drugs (and immigrants) across the Rio Grande. Nothing less than the Mexican army, if that, can restore order.
For now, gang operations in the United States are sporadic. But the border is porus, and if they develop the feeling of invincibility in Mexico, they will try to extend their reach into the United States, at least into those parts where there are many Mexican Americans.
In 1917, the "Mexican Revolution" (actually a de facto civil war between four gangster groups) broke out. They were led by Carranza, Obregon, Zapata, and Pancho Villa. The last was the most feared by the Americans because he crossed the Rio Grande while running away from two of the others (Zapata operated in the South and was most closely aligned with Pancho Villa), thereby "invading" the United States.
It took American army units.under General John J "Blackjack" Pershing to repel the invasion, and put Villa back into his place (south of the Rio Grande). But World War I was raging, and the Mexican "invasion" caused Germany to offer an allance to "Mexico" (then in a state of anarchy from the Civil War), which in turned caused the United States to declare war on Germany. And after having "defeated" the Mexicans, Pershing (a German-American) would go on to defeat the Germans.
If the drug gangs bring about a new "Mexican Revolution" (and this is the opinion of someone who lived in Mexico in 1994), the resulting civil war could cause another invasion of the United States. Then we would have to call "Pershing" to the rescue.