Image copyright Getty Images Image caption A 2016 drug bust in New York City

Nearly half a century ago, Richard Nixon called for an "all-out offensive" on drug abuse. It was the opening salvo in America's longest running war.

Successive presidents took up the call to arms. Arrest rates soared and mandatory minimum sentences sent young men - particularly black men - away for long stretches for low-level offences.

Then as violent crime rates fell under George W Bush and prisons became clogged, prosecutions eased. The war on drugs fell out of fashion. Barack Obama called it "unproductive" and sent memos guiding prosecutors away from pursuing low-level offenders.

Now a new administration looks set to turn back the clock. Attorney General Jeff Sessions likes to reminisce about the aggressive law enforcement of the 80s and 90s and recently labelled cannabis "only slightly less awful" than heroin.

"We will enforce our laws and put bad men behind bars," he said in a recent speech in Richmond, Virginia. "We will fight the scourge of drug abuse."

Image copyright Getty Images Image caption "Good people don't smoke marijuana," said Jeff Sessions

Mr Sessions sent out his own memo last month to prosecutors, instructing them to use "every tool we have", including targeting drug users, in a new crackdown on violent crime. But many of those who fought and studied the first war on drugs say it was a proven failure.

"The people we put away were low-level drug users, not violent criminals," said Tim Longo, who served with Baltimore police between 1981 and 2000.

"We were casting a wide net and catching a lot of people, but most of what we got were guppys and minnows. The people responsible for the homicides, those were the sharks. And you don't catch a shark with a net, you catch a shark with a spear."

Image copyright Getty Images Image caption Richard Nixon declared war on drug abuse in 1971

Between 1980 and 2015, the number of people in prison for drug offences increased more than 10-fold, from 40,900 to 469,545, and the average sentence more than tripled, according to data compiled by the Sentencing Project. The majority of them were low-level offenders with no criminal record.

The idea was to disrupt street gangs, Mr Longo said, but while the approach "might have destabilised the market for a minute, it didn't have any long term, sustainable effect".

A 2013 study published in the British Medical Journal found that since 1990 US drug prices nationwide had fallen while purity increased. And a 2012 study by the University of Florida found that the threat of severe punishment was "generally weak and insignificant" at deterring drug crime or lowering addiction rates.

Dealing long sentences to low-level users and distributors also damaged community relations and made life tough for frontline police, said Norm Stamper, who started as a beat cop in San Diego in 1966 and retired in 2000 as police chief of Seattle,

He recalled low-level drug users being "ripped out of their communities, away from their families, to join the ranks of our mass incarceration problem".

"We were told to hit the streets and make arrests, anything that counted was good," said Mr Stamper, who now works with Law Enforcement Action Partnership, a drug policy advocacy group.

"And we sent a lot of people from our community to San Quentin," he said. "We sent non-violent drug offenders away for 20 years to life under mandatory minimum sentences. Millions of them."

Reports that Jeff Sessions intended to revive these policies made Stamper "heartsick", he said. "We spent $1.5tr (£1.17tr) on this war and drugs are now more available, at higher potency, than when Nixon stood up and made his proclamation.

"Now we have Jeff Sessions quoting Nancy Reagan, saying we've all gone soft. It's all so retrograde it's frightening. We weren't going soft, we were just starting to get smart."

Media playback is unsupported on your device Media caption Drug-related deaths have soared in New Hampshire

Mr Sessions served as a federal prosecutor in Mobile, Alabama in the 80s and early 90s and credits the war on drugs with a steep decline in violent crime that began in 1991 and continued nearly unabated until 2013.

In 2014 the violent crime rate ticked up 3% - the largest one-year increase since 1991. That increase was the result of a "retreat from the aggressive prosecution and incarceration of drug traffickers" the Department of Justice told the BBC.

"The Department has no intention of letting those trends continue to destroy communities," spokesman Ian Prior said.

Earlier this month, Mr Sessions appointed Steven Cook to a top justice department role. A former policeman and prosecutor, Mr Cook is fiercely against the legalisation of marijuana and in favour of maintaining mandatory minimum sentences.

"The federal criminal justice system simply is not broken. In fact, it's working exactly as designed," said Mr Cook last year at an event organised by the Washington Post.

Image copyright Getty Images Image caption An opiate crisis in the US has increased abuse of dangerous drugs like fentanyl

But according to Leo Beletsky, a public health and drug policy expert from Northeastern University, the drop in violent crime had "absolutely zero" to do with the war on drugs. States which declined to lock up thousands of low level drug criminals saw a similar decline, he said.

And a recent study by the Brennan Center, a criminal justice think tank, contradicts the Department of Justice claim. The study examined 14 major theories behind the fall in violent crime between 1991 and 2013, and found that mass incarceration had no effect.

Much more significant, according to the study, were an aging population, changes in income, and decreased alcohol consumption.

A return to the old war-on-drugs style, in the midst of a opiate addiction crisis which has swept the country, would be "a public health disaster", said Mr Beletsky.

Aggressive law enforcement interventions against drug crime had been shown to create economic incentives for more dangerous, more potent alternatives, Mr Beletsky said. A string of recent overdoses have been linked to carfentanil, an opiate 10,000 times stronger than morphine.

"We are at an incredibly vulnerable moment right now," he said, "and the tactics they're talking about would just fuel the crisis."