Secretary of State Marco Rubio has designated eight Mexican drug cartels as "foreign terrorist organizations." The organizations are: 

• Tren de Aragua (also known as Aragua Train); 
• Mara Salvatrucha (also known as MS-13); Cartel de Sinaloa (also known as Sinaloa Cartel, Mexican Federation, Guadalajara Cartel); 
• Cartel de Jalisco Nueva Generacion (also known as New Generation Cartel of Jalisco, CJNG, Jalisco New Generation Cartel); 
• Carteles Unidos (also known as United Cartels, Tepalcatepec Cartel, Cartel de Tepalcatepec, The Grandfather Cartel, Cartel del Abuelo, Cartel de Los Reyes); 
• Cartel del Noreste (also known as CDN, Northeast Cartel, Los Zetas); 
• Cartel del Golfo (also known as CDG, Gulf Cartel, Osiel Cardenas-Guillen Organization); and 
• La Nueva Familia Michoacana (also known as LNFM).

This map gives an idea of where they are dominant.

According to US Northern Command, about one-third of Mexico is under direct cartel control, and cartel presence is pervasive throughout the country.

Designating these groups as FTOs will not, I say again, will not automatically open the door to US military involvement. It would take a "finding" by President Trump to undertake military action against the cartels. Hopefully, that could be done with the assistance of the Mexican Armed Forces, keeping in mind that Los Zetas, aka Cartel del Noreste, was founded by Mexican SOF troops trained by the US Army. Strikes into Mexico without Mexican approval could be very touch-and-go.

While not expressly allowing military action, any person, business, or organization working with the groups would, in the words of the immortal Walter Sobchak, "enter a world of pain." See Foreign Terrorist Organizations - United States Department of State.

What has been amazing has been the pushback Trump has received since he first floated the idea of taking the war to the cartels.

The New York Times (Labeling Mexican Cartels ‘Terrorists’ Could Expose U.S. Companies to Sanctions) and the Wall Street Journal (Designating Drug Gangs as Terrorist Groups Risks Ties With Latin America) concluded, for different reasons, that labeling the cartels as terrorists would hurt us economically and diplomatically. Another New York Times article, which could've been ripped from that paper in 2017, assured us Trump didn't know wtf he was talking about; Opinion | Why Trump Misunderstands the Cartels.

Mexico's Marxist president, Claudia Sheinbaum, is already suing US gun manufacturers (Judge throws out bulk of Mexico's $10bn US gunmakers lawsuit) and threatens to up the ante if the cartels are designated as terrorist groups; see Mexico's president takes aim at U.S. gunmakers if cartels are designated as terrorist groups). 

There is no doubt that the cartels are heavily embedded in Mexico's political and economic life for a lot of reasons, good and bad. There is also no doubt that they will be virtually impossible to stamp out. None of that means we shouldn't drastically increase the costs of their doing business. Time will tell if this action is real or a public relations gimmick. But the CIA has said it is running drone missions over Mexico, presumably to pinpoint drug labs, smuggling routes, and command nodes (C.I.A. Expands Secret Drone Flights Over Mexico), and Mexico's President Sheinbaum supports the effort; see Mexico's president says her government requested US surveillance drone flights. 

Because the terrorist designation was not announced on the first day of Trump's second term in office, I have hope there is a plan behind the Federal Register notice.