Just before the execution unit began slaughtering 16 teenagers at a party in Ciudad Juárez on the US-Mexico border last Sunday night, one of the killers apparently suggested that girls and children be allowed to leave before the killing began. No, said another, "Denles parejo… ya valieron todos" – give it to them all the same.

The massacre of the teenagers made headlines across the world, but it was merely the latest piece of horrific news from Juárez via the only reliable source – that collated daily by Molly Molloy at New Mexico State University for her "Fronterizo List" of subscribers.

Last month, Molloy counted 59 murders in Juárez over the first week and a total of 227 for the month, putting the city on track to beat its own record of 2,657 deaths in 2009. Molloy also gave statistical foundation to journalists' description of Juárez as the "most dangerous city in the world": 192 homicides per 100,000 citizens, well ahead of second-ranking San Pedro Sula in Honduras (119), with New Orleans and Cape Town on 69 and 60 respectively.

There is a grotesque, almost perversely inventive, aspect to the killing going on in Mexico. From another source came pictures of the contents of a vehicle abandoned in Sonora. The torsos, severed heads and limbs crammed inside had been laid out by the authorities across the floor of a hangar.

In the same town, 36-year-old Hugo Hernandez was abducted on 2 January; his body turned up in Los Mochis, Sinaloa, though not in one piece. His torso was in a plastic container in one location, his severed arms and legs were found elsewhere in a box with his skull. But his face was missing; it had been flayed off and was later found near city hall, stitched to a football.

I spent four months last year in Juárez and along the US-Mexico border, writing a book, to be published in the autumn, which among other things tried to answer the question: what on earth is going on?

It would be a fool or a windbag who tried to answer that question conclusively. But certain themes are inescapable: this brutality defines a war very much of its time, the first 21st-century war, because it is, in the end, about nothing. We have lived in a world where Arabs fight Jews, Hutus fight Tutsis, communists fight fascists, Serbs fight Croats, and British and American troops fight Islamist fundamentalists. They do so for a cause, faith or deeply etched tribal identity, however crazy.

But Mexico's war (some do not like calling it a war) has no such purpose. Mexicans are mutilating, decapitating, torturing and killing each other, ostensibly over money and the drug smuggling routes that provide it. But most of the ­violence revolves around the smaller profits of the domestic market and street corner. It is meted out for its own sake. Yes, there are regional and clan allegiances to the states of Tamaulipas, Michoacán or Sinaloa, but they are fluid and subject to far too many whimsical alliances and betrayals for the war to be compared to, say, tribal conflict in Rwanda.

Some would argue that all wars are fought indirectly over money and resources – be they 19th-century wars of empire, or of ideology or religion in the 20th century. But Mexico's war has no ideological pretensions or window-dressing – its only cover is that it was originally fought, like other, lesser, mafia wars, over now diversified product lines that get America (and Europe) high.

But the casus belli now lacks even this as its guiding justification. A brave human rights and migration activist in Reynosa called Mario Treviño, who lives alongside these killers, says: "They're doing it for kudos. They're doing it to show that they can wear this T-shirt by this designer worth this much money. It's like stripes on a military uniform. You walk around and everyone knows what rank you are, because your T-shirt is worth $300.

"It's a system of rank: if you have this T-shirt, you get a cute girl to show off; if you have an even more expensive T-shirt, you get an even cuter girl. But you can't be seen in the T-shirt you wore last year, which has gone out of style – that would mean you hadn't climbed the ladder. Same with the mobile phone and the SUV – you have to have the latest. They're disgusting people, high on amphetamines, but in Reynosa they wear these uniforms of their rank, and they're somebody."

Another wise voice is that of the writer Cecilia Ballí, whose ancestors were once great ranchers around Matamoros and what is now Brownsville, Texas. "People say this is all about money," she says, "but it's about money and something beyond money; it's a social performance, a performance of power, of very male power. It's about being someone, a performance in a place and a country where that was not supposed to be possible."

Mexico's war is fought through You­Tube and mobile phones as well as back-room torture chambers. Cartels and killers use YouTube to threaten rivals and public officials, and boast of their killings, or set up rogue websites to broadcast their savagery. One, based in El Paso and operated by gangs affiliated to the Juárez cartel, received more than 320,000 hits and posted more than 1,000 comments. Images of murders, mutilations and executions are posted on the internet as a blend of sado-pornography, prowess and sick humour.

Neither the left nor the right has managed to muster the slightest resistance. Mexico is a highly politicised society, yet there is no significant trade union, revolutionary or overtly workers' movement against the narco cartels. Similarly, there is no sign of a rightist paramilitary, ­fascist or vigilante movement for law and order; no Mussolini figure to the right of President Felipe Calderón.

Instead, a war that is quint­essentially materialist and largely male meets resistance from two quarters that do not belong to conventional politics – religion and women. Though the Catholic church remains equivocal, priests on the ground face down the narcos and have been executed for doing so; rehab centres attacked by the narcos are run by born-again evangelists, often former addicts and gang members themselves.

The women fight as individuals, through organisations and in the home. In this deeply religious country, even the narcos have "sanctified" their war through the nihilistic cult of "Santisima Muerte" – the goddess of death.

After several years reporting from the border, and for the best part of the past year on the narco war, I feel in some way more confused than when I started. Not least because a "post-political" war is only meeting resistance from "pre-­political" religion and the clergy – something that baffles a secular mass media always looking for political or military solutions that serially fail.

In 1988, long before the worst, a photographer still working in Ciudad Juárez called Julián Cardona and the writer Charles Bowden produced a book entitled: Juárez: The Laboratory of Our Future. The future has arrived. And it appears to offer an experiment in a new form of mass violence with no purpose other than spectacle.

**Amexica: War Along The Borderline by Ed Vulliamy will be published in ­September by The Bodley Head, London; Farrar, Straus and Giroux, New York; and Tusquets, Madrid