Did they do that pointless exercise to show who was boss in those parts? It's hard to say. CNN had the most detailed report, beginning with :

A couple days ago, Mexican troops reportedly came over into U.S. territory near El Paso in an unmarked vehicle on April 13, and disarmed the U.S. soldiers on the U.S. side, sent by President Trump to support U.S. border protection operations. Weirdly, they took the U.S. soldiers' weapons they'd confiscated, and then placed them into the soldiers' own vehicles and left.

Two US soldiers were questioned by Mexican troops earlier this month while conducting a surveillance operation on the US side of the southern border, two US defense officials tell CNN. "On April 13, 2019, at approximately 2 p.m. CDT, five to six Mexican military personnel questioned two U.S. Army soldiers who were conducting border support operations in an unmarked (Customs and Border Protection) vehicle near the southwest border in the vicinity of Clint, Texas," US Northern Command told CNN in a statement. "The US soldiers were appropriately in US territory" during the encounter, the statement added. During the incident, the Mexican soldiers pointed their weapons at the US troops, removing a soldier's sidearm and returning it to the unmarked US vehicle, the officials said.

That's an egregious violation of U.S. territory and under most circumstances would have been grounds for firing back. The report said the soldiers followed procedures, however, recognizing that there wasn't a war on, and allowed the Mexicans to disarm them to prevent an escalation. No foul on their part. That said, the facts are now out: The Mexicans were in the wrong, they were invading our territory, witting or not, they treated our soldiers like miscreants, and now they owe us an apology. That's the appropriate response. That's what decent nations do. I find no evidence they've given one.

The CNN report explains the matter as a simple mixup, with Mexican soldiers not knowing where they were, but there's some grounds for skepticism.

Start with how the border near Clint, Texas is delineated. The dividing line is a river - the Rio Grande. The U.S. soldiers were standing on the U.S. side which came before a border fence yet was still on the right side of the river. The Mexicans would have had to have crossed a river to get to where they could intercept a U.S. soldier and it's quite unlikely you could cross a river, even a shallow one, without knowing it.

CNN reports that the river can be dry at times, and there's lots of brush in the area, meaning, it's easy to get lost. But there's also an argument to be made that the brush itself would make the river's location knowable just by observation, and even more so with basic soldier compass skills. At a minimum, the Mexicans didn't have those, which is an embarrassment on top of a blunder.

But I don't think that's the only possibility as to what happened.

Wikipedia reports that the Rio Grande tends to run dry at the Presidio, Texas point in the river, which is not far from Big Bend. Clint , Texas, where the incident happened, is a five-hour-drive 250 miles northwest . If Wikipedia is right, the river should have still been running.

I also had a look at Rio Grande river levels in mid-April, checking with data from the National Parks Service. Average river depth in April is three feet, but that is just an average, it could be quite shallower near Clint, Texas.

Here are the water readings from multiple points on the Rio Grande, according to the National Park Service - this is their list with their links