Last month, a police truck burned on the side of the road in Zihuatanejo, Guerrero, with flames melting the interior and the four bodies, three of them murdered federal police officers, that lay inside.

Yet, the brutality barely registered in the region’s collective consciousness beyond a few cursory news stories. These were just four more deaths in a country that is once again living through a wave of violence.

Stephanie Leutert

The last time that Mexico’s murder numbers spiked so dramatically was over six years ago, not long after former President Felipe Calderón began his well-known confrontation against the country’s organized criminal groups, before falling through the start of the Enrique Peña Nieto presidency in late 2012. Yet the much welcomed drop in murders – which has defined Mexico’s security situation over the past few years – has now bottomed out. And today’s homicide numbers are creeping upward, across ever-larger swaths of the country.

Some of the reasons behind this second spike in violence are the same as in previous years. First, the Mexican government still faces its perennial challenges of strengthening a weak rule of law, and tackling collusion among local authorities and criminal groups. Despite federal efforts to reform the judicial system, professionalize the police, and reduce corruption, impunity remains high, and trust in the authorities is still low.

Second, the Mexican government has barely changed its security strategy over the decade long offensive. President Peña Nieto entered into office with a rhetorical shift on security issues, but the current strategy continues to call for Calderón-era confrontation with criminal groups and prioritizes the targeting of kingpins. Law enforcement and security officials have been successful with Calderón killing or capturing 25 of his 37 most wanted narcos and Peña Nieto plucking off 105 of his top 122.

Yet, each downed narco creates more than a photo-op. The relentless pounding of top leadership also breeds an underworld power vacuum, where rivalries, ambitions, and greed all inflame intra-cartel backstabbing and leave splinter groups locked in bloody battles. These groups have adopted gruesome tactics and ventured into additional activities to beef up their revenue – such as extortion or kidnapping – often far more damaging to Mexican communities than standard drug trafficking.

There are dozens of these groups, but the standout is the Cartel Jalisco New Generation (CJNG), which has steadily expanded its territorial reach since 2010. Its surge to power has placed it comfortably “among the most powerful drug trafficking organizations in Mexico,” according to the U.S. Treasury, and it’s also quickly becoming among the most murderous.

There are also market forces at work in driving up the violence. The United States’ prescription drug epidemic has pushed addicts who can’t get pills into cheap heroin to fulfill their opioid cravings. Either sensing a market opportunity or strategically boosting the demand, Mexican cartels have capitalized on the health crisis, quite literally fighting one another to provide the heroin supply.

However, one thing that looks dramatically different this time around is the promise of a U.S.-Mexico bilateral security partnership. In 2007, as Calderón began his push against criminal groups, Mexican officials worked with their American counterparts to form the Mérida Initiative and formalize the binational working relationship on security issues.

The Mérida platform earmarked $2.5 billion in U.S. funds over eight years (of which $1.5 billion was released) to complement Mexico’s $79 billion in security investments over the same period. Over time, the Mérida Initiative’s mandate evolved, but it always focused on supporting Mexico’s efforts to implement a more professional and institutionalized security response.

During this second wave of violence, the Mérida Initiative stands on shaky policy ground. President-Elect Donald Trump has largely adopted a go-it-alone approach for tackling regional crime and violence. When combined with Trump’s “America First” motto and a distaste for foreign aid, the Mérida Initiative and overall bilateral security cooperation is unlikely to look the same or even exist on a recognizable scale in the coming years.

Slowing Mexico’s bloodshed will continue to require a comprehensive security policy approach. It will also require international cooperation – whether in the form of the Mérida Initiative or otherwise – as the market forces and criminal groups stretch well beyond Mexico’s borders. Yet as Mexico’s murder levels climb the response will need to be quick and multi-faceted, addressing and tamping down on the flames of violence that are once again burning across the country.

Editor’s Note: The above guest column was penned jointly by Antonio Garza, the former U.S. ambassador to Mexico, and Stephanie Leutert, Mexico Security Initiative Fellow at the University of Texas at Austin.