Guzmán Decena was born in a poor family in Puebla and joined the military as a teenager to escape from poverty. While in the military, he was a talented and bright soldier, earning a position in the Special Forces of the Mexican military by the mid-1990s. During his military career, Guzmán Decena received counter-insurgency training, acquired skills in explosives, and learned how to track down and apprehend his enemies from an elite combat group trained by the U.S. Special Forces and the Israel Defense Forces.
He began to take bribes from the Gulf Cartel while still serving in the military, but eventually defected to work full-time for the criminal organization in 1997. For years he recruited other members of the Mexican Armed Forces to form Los Zetas, the armed wing of the Gulf Cartel, along with Cárdenas Guillén.
Guzmán Decena was born in a poor village in Puebla, Mexico on 13 January 1976, and finished middle school and high school before joining the Mexican military to escape poverty. His talents and aggressive behavior earned him a position with an elite Mexican military group called Grupo Aeromóvil de Fuerzas Especiales (GAFE), originally trained in counter-insurgency tactics for the Zapatista uprising in 1994 and for locating and apprehending members of Mexico's drug trafficking organizations. Guzmán Decena reportedly received military training from the Israeli special forces. His training came into practice after more than 3,000 Zapatista rebels seized several towns across the southern state of Chiapas in 1994. The rebellion was a symbolic rising against poverty and the single-party rule of the Institutional Revolutionary Party (PRI), and many rebels took arms; the Mexican government, however, sent in the GAFE to put down the Zapatistas. Within hours, 34 rebels were killed and three others were captured by Guzmán Decena's counter-insurgency group. Their bodies were then disposed on a riverbank – with their ears and noses sliced off.
Now one of the brightest, highly trained, and bloodiest members in the GAFE, Guzmán Decena was sent to the northern state of Tamaulipas. While operating as the security chief in the city of Miguel Alemán, Tamaulipas, he was recruited by Cárdenas Guillén's cartel. Investigators say that Guzmán Decena first worked with the Gulf Cartel by taking bribes from Osiel Cárdenas Guillén and turning a blind eye on the drug shipments of the cartel. Such payments were typical among military commanders, but while soldiers had often accepted bribes from the drug lords, it was not common for them to defect from the army and join their ranks. Bribes were seen by soldiers as "benefits" to their job, and officers stood firm to the idea that they were protectors of the Mexican people. Guzmán Decena, however, shattered that model and left the military in 1997 to work full-time with a drug trafficking organization. According to the British journalist, Ioan Grillo, it is still unclear why Guzmán Decena defected from the army to become a "narco-mercenary." A common explanation is that Guzmán Decena left the military in search of a higher payment, seeing that many cartel members lived ostentatiously and earned more in a year than a GAFE member earned in a lifetime. Nonetheless, he would have also lived comfortably as a successful GAFE member in the army. And by joining the Gulf cartel, he was becoming a fugitive and increasing his chances of being arrested or killed. Hence, a crucial factor in his defection may have been the seismic change of Mexico's transition to democracy and the tearing rule of the PRI. The "new Mexico" and the democracy that came with it was feared by many soldiers who had made abuses during the old regime. Mounting pressures arose from the families of the "disappeared" who made marches in Mexico City, and many military officers were found guilty in courts-martial for human right abuses and corruption. For years, some military generals took bribes from the cartels; amid the turmoil, Guzmán Decena acknowledged that he was better off outside the system and as a leader of Los Zetas.