The Mexican Senate on Monday authorized the entry into the country of a group of 139 U.S. military personnel. The operation, intended for training purposes, will take place at the Santa Gertrudis military base in Chihuahua between September 20 and 30.

The troops’ arrival comes just months after a political and diplomatic controversy over the presence of CIA agents in Chihuahua. It also takes place as Mexico and the United States continue to cooperate on security issues, while concerns over interference from the Donald Trump administration continue to shape the bilateral relationship.

The Mexican Senate said that the arrival of U.S. troops, a routine exercise within the framework of cooperation between the two countries’ armed forces, “will not only have a positive impact on strengthening individual and collective preparedness, but will also contribute to improving bilateral military cooperation mechanisms between both nations.”

The request submitted by Mexican President Claudia Sheinbaum, designated as an “urgent resolution” for the entry of the military personnel, was approved unanimously by the Senate with 75 votes in favor.

The U.S. contingent is made up of soldiers from the 101st Airborne Division of the United States Army. They are scheduled to arrive at the training base next Thursday and have authorization to remain in Santa Gertrudis, located 320 miles south of the border, until the end of September. They will enter Mexico carrying weapons aboard a U.S. Air Force aircraft.

This is the fourth time this year that the Senate has authorized the entry of troops from its northern neighbor, although it is the first deployment involving more than 100 personnel. The first request from Sheinbaum came in February, when the chamber approved the entry of 19 foreign troops, with 105 votes in favor, to take part in “a specialized training exercise” in collaboration with the Mexican Navy’s Special Operations Unit in the state of Campeche.

That same month, lawmakers also authorized the entry of 12 members of the U.S. Army’s 7th Special Forces Group (Airborne) for training activities. The most recent authorization came in April, allowing 12 U.S. service members to help enhance “the capabilities of special operations units” between July 15 and December 15, 2026.

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A threat, the nature of which has not been disclosed, has led U.S. authorities to restrict the movement of their personnel in Nogales, Sonora, where officials will only be allowed to make trips deemed essential and during daylight hours. The restriction applies to one of the major cities along the U.S.-Mexico border and was announced without U.S. authorities revealing what specific information prompted the measure. Sonora authorities, for their part, later stated that they had no information indicating a specific risk to the public, although they will maintain preventive security operations.

The U.S. Embassy and consulates in Mexico issued the alert on Saturday, without providing details about the nature of the threat or explaining what information led to the modification of security measures in Nogales.

“Due to threat information, U.S. government activities in, and travel to, Nogales have been restricted to essential movement during daylight hours only,” stated the U.S. Consulate General in Nogales. “We are alerting U.S. citizens of this threat and change in posture.”

The warning does not specify how long the restrictions will remain in effect. Nor does it clarify whether they could be extended to other activities or areas.

The U.S. government has urged its citizens who are in Nogales or planning to travel to the city to take extra precautions and remain alert to their surroundings. Recommendations include seeking shelter if a security incident occurs, informing family and friends of their situation, following the instructions of Mexican authorities, and checking local media for updates.

The warning comes in addition to Sonora’s existing classification within the U.S. travel advisory system. The state is currently at Level 3, which advises U.S. citizens to reconsider travel due to security risks. The new alert regarding Nogales does not change this level for Sonora, but it does serve as a specific warning about the situation in the city.

After the United States issued the alert, the Sonora State Security Council reported that it had contacted U.S. consular officials. State authorities said they had no information indicating a concrete risk to the public. Nor did they identify a specific threat related to the U.S. warning. Despite this, they noted that they would continue preventive security operations in Nogales.

The lack of public information leaves the main question surrounding the alert unanswered: what threat led Washington to restrict the movements of its officials? For now, the United States has only stated that the decision is based on “threat information,” without providing further details.

Republican Congressman Andy Biggs of Arizona urged Americans in or around Nogales to follow the government’s recommendations. “For Americans in or near Nogales — or who may have family members or friends there: please heed the guidance and warnings from the U.S. government,” Biggs wrote on X. “We are praying for the safety of U.S. citizens and personnel who may be in harm’s way,” he added.

The warning also comes amid growing tension surrounding Sonora and its governor, Alfonso Durazo. In June, the U.S. government revoked the visas of Durazo and Tamaulipas Governor Américo Villarreal amid allegations of ties to organized crime — claims that both state governments denied. Days later, Durazo stated that he held a valid visa and also denied having been identified as a Washington informant against other members of the ruling Morena political party, following a New York Times article that prompted a public response from President Claudia Sheinbaum and the two governors.

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