Heads of state from across the Americas will gather this weekend for the sixth Summit of the Americas. Agenda item number one: the war on drugs. The decades-old policy, led by the US, has been a disaster – costing tens of thousands of lives, billions of dollars and achieving next to nothing. Now a group of Latin American leaders is calling for a radical departure: the legalisation of drugs.

Something's got to change

The war on drugs first took shape under US President Richard Nixon (the man who took office by exploiting white Americans' fear of their black neighbours – known as the "southern strategy"). A combination of tough domestic policing, regional military aid and direct, often violent intervention by US special forces, the drug war has sputtered on for four decades. And by every reasonable metric it has failed. According to the US government, both demand for and availability of drugs are on the rise. In the US's overcrowded prison system, non-violent drug offenders make up about 25% of the population.

In Latin America the costs are even higher. Far from bringing stability, the war on drugs has taken nations to the brink. In Mexico, the principal trafficking gateway into the US, more than 50,000 people have been killed in drug-related violence since the end of 2006 (compare that to 12,793 Afghan civilians killed over the same period). In recent years the violence has spilled over Mexico's borders: Guatemala's murder rate is twice that of Mexico. Just south, in Honduras, it's the highest in the world.

No wonder that regional leaders, from left and right, are calling for change. Guatemala's conservative president, Otto Pérez Molina, a former head of military intelligence, has openly called for drug legalisation as a way of undermining powerful drug cartels. In late March, he hosted a meeting with other Central American presidents to discuss the proposition.

A rare opportunity

It's fitting that the Summit of the Americas will be held in Colombia – a country once synonymous with narcotics violence. Colombia is now one of South America's rising stars and its president, Juan Manuel Santos, is looking to become a regional leader. Santos is a close ally of the US, but he's also said that he's open to the idea of legalising drugs.

Leaders from the US and Canada are bound to oppose any real change of course, particularly in a year when the US president faces re-election. But given the summit's symbolic location, its host's willingness to talk, and Otto Pérez's recent call, it seems certain that drug policy will be front and centre – and exploring new ways forward will be unavoidable. President Obama and Vice President Biden will be at the meeting, and Obama has already admitted that legalisation is a "legitimate topic".

Avaaz members have worked hard to end the costly and destructive war on drugs. This might be our best chance to beat back the murderous drug cartels and put failed policies to rest.

Take action:Sign this petition to support the Guatemalan president's call to legalise drugs.