NEW YORK - The US diplomat most associated with the invasions of Iraq and Afghanistan says that American policymakers need to learn the lessons of the recent past as they weigh military options for the future, including for Syria and Iran.

Remember the law of unintended consequences; recognise the limits of the United States’ actual capabilities; understand that getting out of a conflict once you are in can often be as destructive for the country as the original conflict, Ryan Crocker, the retiring US diplomat, currently the ambassador in Kabul, said in an interview with The New York Times.

“You better do some cold calculation, you know, about how do you really think you are going to influence things for the better,” said Crocker, 63.

Crocker, one of the pre-eminent American diplomats of the past 40 years, said he could not help keeping his mind at work on the crisis spots that have defined his career - in Pakistan and Afghanistan, Iran and Syria, Iraq and Lebanon. He will retire at the end of July after a career that began as the last American troops were leaving Vietnam and is ending as the curtain closes on an era of American state-building that has mostly fallen short of the results policy makers had hoped for, the Times said.

In the years ahead, Crocker sees, if anything, an increasingly fraught foreign landscape in a world set afire by war and revolution, a chapter bound to frustrate the best intentions and most sophisticated strategies of the US, the paper said.

Although he speaks Arabic and has spent a lifetime immersed in the Arab world and Afghanistan, Crocker is deeply sceptical that Americans on foreign soil can be anything other than strangers in a strange land.

“We are a superpower, we don’t fight on our territory, but that means you are in somebody else’s stadium, playing by somebody else’s ground rules, and you have to understand the environment, the history, the politics of the country you wish to intervene in,” he said.

“Although publicly Mr Crocker has sometimes presented the glass as half-full when assessing the situation in foreign countries, fellow diplomats say that his private analyses tend to be stark and unromantic — a vision shaped by his 38 years of experience in which he confronted over and over the limits of American power and the hostility of many in the world to what the United States stands for,” the newspaper said.

On the deteriorating situation in Syria, Crocker said “I worry greatly that the minorities, the Alawis and the Christians, are going to be in for a very awful time.”

He added that he fears as well that if Muslim hardliners take over, ‘the repercussions for Syria, for Lebanon and Iraq, I think, can be pretty serious’.

As for the United States’ ability to sway the situation, he was again reflective. “We’ve been writing memos to policy makers with the subject line ‘Levers on Syria’ for decades,” he said. “Well, you know, the reality was those levers didn’t exist.”

Now, he added: “I’m not sure we can do much to influence it.”