Rambo shows me the marks around his wrists, left after being chained up by the Taliban for more than five months.

"When the US marines leave, we will have to fight to protect our people," he says.

Rambo is a policeman in Kajaki district in Helmand province. His real name is Omar Gul, but he only answers to the moniker of the action-movie hero made famous by Sylvester Stallone.

Rambo hates the Taliban. He somehow managed to shoot his way out of their custody. Now he wants revenge. That drive makes him stand out against his fellow police officers and highlights the challenges Afghan security forces will face as US and NATO troops pull out of Afghanistan.

The other police officers we met in Kajaki and elsewhere across this country are not motivated like Rambo.

They just need the money. There is no real loyalty to the leadership of one of the most corrupt countries on earth. So, when the foreign troops leave, it is hard to imagine poorly paid and poorly equipped police officers standing and fighting against determined Taliban.

Kajaki has only been under central government influence for a little more than seven months. Out on patrol with troops of the US Marine Corps, we walk past fields of poppy stalks. Their sap has just been harvested for opium. The Americans do not destroy this harvest because, they say, they do not want to alienate the local population.

One US soldier tells me that this area was so detached from the Afghan government that, before they arrived here in October, local farmers did not know that poppy cultivation is illegal.

Hard work

We are embedded with a detachment of 18 US soldiers. They live with Rambo and his colleagues in a police compound in Kajaki's bazaar. The Americans are here to mentor the Afghans, to get them ready to go it alone in perhaps just a few months. It is hard work.

The Marines Corps are part of arguably the best-equipped military machine in the world. They have heavily armoured. mine-resistant. ambush-protected vehicles, MRAPS, which can withstand roadside bombs.

They have sophisticated spy cameras fastened to blimps that float high above towns and villages. They can call in air support if the going gets really tough. The US soldiers we were with were fit, disciplined, committed and professional men.

The Afghans, meanwhile, must manage with Ford Ranger pick-up vehicles, AK-47s, and not much else.

When we ask the US military how the Afghans will cope when they lose foreign military support, the answer is "local knowledge". Which means building local relationships to gather intelligence to maintain stability.

On a walking patrol through Kajaki’s bazaar we see this strategy in action. The police talk to the locals while the Americans look on from a distance. It all seems very friendly. But when we approach shopkeepers, some, out of earshot of the police, complain of official corruption. Others say they are too afraid to talk.

Stability in Kajaki seems fragile, and not every police officer is like Rambo.