The State Department advised Americans this week to defer “non-essential travel” to vast stretches of Mexico, warning that 14 of the country’s 31 states are so dangerous that visitors should avoid them if at all possible. For four other states, it counseled caution or extreme caution.

The travel warning is at once broader, more detailed and more alarming than the previous one for Mexico, issued in April.

The new warning became public as Mexican troops announced Thursday that they had seized 15 tons of pure methamphetamine outside Guadalajara — an amount equal to half of all meth seizures worldwide in 2009.

State Department travel warnings are based on internal guidance that embassies and consular offices use to decide where it is safe for U.S. diplomats and federal employees to travel, so they often err on the side of caution.

Still, this one, issued Wednesday evening, is sweeping. To begin with, it warns against all but essential travel across most of the states along the U.S.-Mexican border: Tamaulipas, Nuevo Leon (except the city of Monterrey, where caution is advised), Coahuila, Chihuahua and Sonora.

Monterrey is Mexico’s version of Houston, a boom town filled with corporate boosters and an aspirational middle class. But the State Department discloses that since September 2010, the U.S. Consulate General in Monterrey became an “unaccompanied post with no minor dependents of U.S. government personnel permitted.” The warning concludes: “The level of violence and insecurity in Monterrey has increased, illustrated by an attack on a popular local casino in August that resulted in 52 deaths. One U.S. citizen was injured in that incident. Local police and private patrols do not have the capacity to deter criminal elements or respond effectively to security incidents.”

Ciudad Juarez, in Chihuahua, merits “special concern,” the warning says, advising that the border city “has one of the highest murder rates in Mexico” and that “three persons associated with the Consulate General were murdered in March 2010” there.

Moving south, also on the no-go list for all but essential travel: Sinaloa (except the Pacific Coast resort of Mazatlan), Durango, Zacatecas, Aguascalientes and San Luis Potosi, where two U.S. Immigration and Customs Enforcement agents were ambushed and one was killed a year ago.

This means a visitor who wants to drive from the United States to Mexico City has no viable route that would be in accord with the U.S. guidelines.

If you do drive, the warning says, remember: “TCOs [Transnational Criminal Organizations] have erected their own unauthorized checkpoints, and killed or abducted motorists who have failed to stop at them. You should cooperate at all checkpoints.”

The State Department also warns against travel in Jalisco along its borders with Michoacan, another no-go, and Zacatecas.

Some of the State Department’s advice can be chilling. For example, it warns travelers to avoid much of the southern Pacific states of Nayarit and Guerrero, except for the popular beach resorts of Riviera Nayarit, Ixtapa, Zihuatanejo and Acapulco. But watch your step even there, the department said: “In Acapulco, defer non-essential travel to areas further than 2 blocks inland of the Costera Miguel Aleman Boulevard, which parallels the popular beach areas.”

And skip the highways: “Flying into the coastal cities in southern Guerrero remains the preferred method of travel.”

The advisory does note that “millions of U.S. citizens safely visit Mexico each year for study, tourism, and business, including more than 150,000 who cross the border every day.” Still, it says, U.S. travelers should be aware of Mexico’s efforts against “TCOs [trans­national criminal organizations] which engage in narcotics trafficking and other unlawful activities” throughout the country.

Mexico is a country of 110 million people, so the odds of running into trouble are low. The number of U.S. citizens reported to the State Department as murdered in Mexico increased from 35 in 2007 to 120 in 2011.

Where to go? Much of the Yucatan Peninsula is free of murder and mayhem. No advisory is in effect for the states of Chiapas, Tabasco, Campeche, Yucatan and Quintana Roo. Good to go, too, are the popular art and food destinations of Oaxaca and Puebla.

Also on the safe side of the ledger: Mexico City.