In this extract from his latest book, Little America, Rajiv Chandrasekaran exposes the ways in which the supposed allies were at loggerheads when it came to strategy in Afghanistan

American marines and British civilian advisers were waging two wars in the hilly northern half of Helmand province by mid-2010: they were fighting the Taliban – and each other.

The quarrelling among the allies began with the arrival that spring of more marine forces as part of the US surge, but the tension had its roots in the summer of 2006, when British commanders had decided to establish outposts in the districts of Sangin and Musa Qala that the Taliban quickly besieged and nearly overran.

In Musa Qala, the first wave of British soldiers almost ran out of ammunition after three months of harrowing combat. With no easy way to resupply them – it was too risky to drive convoys or fly dual-rotor Chinooks, the only helicopters the British had – desperate commanders accepted a dubious offer from a tribal leader in the district: if British troops would leave, the leader said, so would the insurgents, whereupon residents would take charge of security. The British departed in a convoy of local freight trucks.

The truce was short-lived, and by the following February, hundreds of Taliban fighters had recaptured the area, prompting the British, aided by the US Army's 82nd airborne division, to conduct a massive operation in late 2007 to wrest back control of the district centre. They succeeded in pushing the Taliban out of town. Then the British stopped. They established front lines about four miles north and south of the town centre; everything beyond was insurgent country.

There was no deal to be made in Sangin, so British commanders sent more forces. Still they did not have the manpower to establish a decisive advantage.

The insurgents set up bomb-making factories in a valley that hugged the source of the Helmand river and they struck an informal mutual defence pact with drug barons who ran a network of opium-processing labs in hillside hamlets. The British commanders made matters worse by spreading their troops across several small outposts along the valley, condemning them to a Sisyphean mission: they would clear insurgents from small parts of the district, but then they had to move on. The Taliban would seize them back, forcing the Brits to attack the same areas again and again. Sangin's lush wheat fields and dense poppy groves soon became killing fields. From 2006 to 2010, Taliban bombs and bullets in the district claimed the lives of more than 100 British troops – about a third of the country's total war dead in that period.

Soon after the Musa Qala truce fell apart, the top US and Nato commander at the time, General Dan McNeill, told a visiting American official that the British had "made a mess of things in Helmand". In January 2009, Helmand's Afghan governor, Gulab Mangal, made a brief visit to Sangin, where he discovered that insurgents were operating with impunity no more than 500m from the district centre. Mangal was incensed when British troops told him it wasn't safe to venture to the bazaar – or anywhere farther than 200m from the main British encampment. "Stop calling it the Sangin district and start calling it the Sangin base. All you have done here is build a military camp next to the city," he complained.

Mangal grew even angrier when the Afghan army commander in Sangin and the district governor told him that British troops "were searching compounds, walking on the roofs of homes, and treating the local population badly – including pointing weapons at people and going into areas where women were working," according to a US State Department cable describing the visit.

The dissatisfaction with the British extended up to Kabul. Late in 2008, Afghan president Hamid Karzai questioned the effectiveness of the British during a meeting with US senators John McCain, Joe Lieberman and Lindsey Graham. He related an anecdote about a woman from Helmand who had asked him to "take the British away and give us back the Americans".

British officials insisted that their problems in the province stemmed from a lack of manpower. Unlike the Canadians, who ceded parts of Kandahar province to the Americans in 2009 only grudgingly, British commanders were eager to give up responsibility for large swaths of Helmand. To avoid losing face, the MoD wanted a discreet bailout. The British sought a few thousand US troops to help in the central and northern parts of the province. That would allow the British to hand over [the districts of] Garmser and Nawa and avoid dealing with Marja. They would concentrate on Lashkar Gah, the community of Nad Ali and the northern districts of Musa Qala and Sangin. The British also insisted on retaining control of the reconstruction office in Lashkar Gah, which took its orders from the Foreign Office in London, not the Nato headquarters in Kabul.

The decision to send the hard-charging US marines to Helmand instead of Kandahar upended British hopes for a delicate balance between the allies. The top marine commander in Afghanistan was Larry Nicholson, a deft practitioner of modern warfare whose out-of-uniform interests included listening to Katy Perry and watching Downton Abbey, and when his brigade arrived, there were more Americans than British in the province. Nicholson quickly grew irritated with the British approach to countering the insurgency.

He abhorred the establishment of front lines that they did not cross, and he recoiled when he saw how Afghan soldiers were segregated in camps on British bases. He advocated genuine partnership, not a vestigial colonial attitude toward the natives, and that meant eating and living together. What riled him the most, however, was the British reconstruction team. The team's members had their own views about which parts of the province merited military attention, and those did not always mesh with Nicholson's.

That was to be expected, because the British had been trying to triage a shorthanded mission for three years. But the United States had injected 10,672 marines into Helmand, and Nicholson wanted the voting rights that came with being a majority shareholder.

Tensions flared during the marines' first operation – the push into Nawa, Garmser and Khan Neshin. The British thought it was a fool's errand to travel deep into the Desert of Death and conduct a full-scale counterinsurgency mission in Khan Neshin. What mattered to Nicholson was that Governor Mangal wanted the marines to head there. When state department official Marc Chretien visited the reconstruction office two months after the operation commenced, feelings were raw.

"Your marines seem to have exceeded the ops plan," a British lieutenant colonel told him.

"Well, of course," Chretien replied. "They're marines. They're dogs of war. It's what they do."

When Chretien had informed a three-star British general who was visiting Anbar province that he was heading to Helmand, the general had said: "We want a marriage with you Yanks in Helmand, not a date rape." Chretien took the request to heart, and he repeated the quote to Colonel Mike Killion, Nicholson's top operations officer, before a meeting they had with the head of the reconstruction office in September 2009. Killion was an expletive-spewing marine with a tin of snuff in his pocket who oversaw the brigade's daily combat missions.

"Check," Killion said to Chretien as they walked in to meet with Hugh Powell, an Oxford-educated diplomat who ran the reconstruction office and was supposed to be of equal rank to Nicholson and the British general in the province. The meeting began with a series of pleasantries and then a bit of history. "Helmand was an idyllic place," Powell said, "and then you marines arrived."

Killion believed the province had been overrun by the Taliban because of British appeasement. "Look, buddy, it's not hard to be idyllic if you're sitting on your ass," he growled. "We're here to win."

Davd Cameron, then opposition leader, chats with British army commanders during a visit to Afghanistan in December 2009. Photograph: Stefan Rousseau/AFP/Getty Images

The meeting degenerated. So did the relationship. When the marines pushed into Now Zad that December, Powell's successor, Lindy Cameron, refused to provide the same sort of reconstruction resources accorded to other districts in the province because she didn't think it made sense to try to rebuild a city that by that point had been completely abandoned by its population. "They stiffed us," Nicholson seethed to me at the time. "So much for all of us being on the same team."

Cameron had a legitimate argument, but the marines didn't want to hear it. They had written off the reconstruction office as hopelessly disconnected from the war, a view that was reinforced by the frequent parties and social events that occurred on the office's compound in Lashkar Gah. It wasn't as wild as the US embassy in Kabul, but Nicholson's officers were nevertheless incredulous when they learned that the office had held an alcohol-sodden "Lash Vegas Pimps and Hos" bash while the marines were struggling to pacify Marja.

Disagreements also erupted in Musa Qala after marines replaced British soldiers in early 2010. Within 48 hours, the Americans had punched beyond the northern front line and seized a town that had long been a Taliban stronghold.

Then marine units began targeting insurgents well beyond the old southern line. "They didn't pursue the Taliban," the marine commander in the district, Lieutenant Colonel Michael Manning, said of the British. "We'll go after them."

When I visited Musa Qala in July 2010, Manning made little effort to conceal his frustration with the pace at which representatives from the reconstruction office were rebuilding the area. Musa Qala's grand mosque, which had been destroyed in the 2007 military operation, was supposed to have been fixed two years earlier. It remained a giant hole in the ground. The British had also pledged to construct a bridge over a riverbed that floods every winter, forcing people to rely on ferries. "They were here for four years," another marine officer said as we gazed over the still bridgeless river. "What did they do?"

British officials insisted that construction of the mosque and the bridge had been delayed because they had been teaching the Afghan government how to take charge of such projects itself. "The US approach is focused on getting it done. Our focus is on building up the government to deliver," a British government official in Helmand told me.

The disputes between the allies soon led to a more fundamental difference over war strategy: the marines were bent on expansion; the British were intent on retrenchment. By the spring of 2010, about 20,000 US troops, mostly marines, were stationed in Helmand, compared with about 9,000 British.

The marines wanted to push south to the town of Barham Chah on the Pakistani border, and west to neighbouring Nimruz province, which abuts Iran. Neither foray seemed in keeping with General Stanley McChrystal's counterinsurgency mission. Barham Chah was tiny, and the bad guys in whom the marines were interested – insurgents and drug smugglers – often drove through the desert, bypassing the town. Nimruz was largely sand. The Nato command deemed it so strategically unimportant that it was one of just four provinces without a reconstruction team. But the marines saw danger – and potential.

Their intelligence reports indicated that Taliban fighters were using the northern part of the province to stage attacks in Helmand. They began constructing a vast base on the province's north-eastern edge. The original plans for the encampment envisaged two airstrips, an advanced combat hospital, a post office, a large convenience store and rows of housing trailers stretching as far as the eye could see. Nicholson expected more than 3,000 marines – one-tenth of the surge troops – to be based there by mid-2010. But the marines' ambition did not factor in British fatigue.

As British casualties mounted in 2009 – that July was the bloodiest month yet for British forces in Helmand – the public's weak support for the war dipped further at home, prompting calls for a change in strategy. Some in the Labour party called for a full-scale withdrawal by the end of the year. It was clear that Britain's stance in Helmand needed to shift if Gordon Brown wanted to prevent an outright revolt. His government's first step was to inform the Obama administration that it would not increase forces, even nominally, in tandem with an American surge. Then the MoD instructed military commanders to work up secret plans to hand over Sangin and Musa Qala to the Americans and consolidate in less dangerous parts of the province.

The British desire to give up Sangin became clear in December 2009 when David Cameron, who would become prime minister in a few months, visited Helmand. His countrymen, he said, were "spread too thinly". By then, 245 British soldiers had been killed in Afghanistan. Another statistic also weighed on British leaders: although they had 30% of the troops in Helmand, they were responsible for 70% of the population.

When Cameron met with Nicholson and his political adviser, Kael Weston, the top British commander in Helmand, Brigadier General James Cowan, began laying the groundwork for the shift by emphasising the British sacrifice. Cowan said he had recently asked the British field hospital next to Camp Leatherneck in Helmand, which treated both Americans and British, how much blood it had transfused in the past month. The answer, Cowan reported, was that far more blood had been used on the British – because they had so many casualties – than on the marines.

An awkward silence descended on the room. Cameron said nothing. After 10 seconds, Weston spoke up. "There's been enough blood shed by all of us here," he said.

The body of a British soldier killed in Helmand is brough home at RAF Lyneham, July 2009. Photograph: Richard Watt/MoD/PA

The following month, when [Labour] foreign secretary David Miliband visited Helmand, an aide passed him a note during his meeting with Nicholson and Weston. Two more British soldiers had been killed in Sangin. It was at that moment that Weston grasped the stakes for both nations: the British could no longer sustain those sorts of messages, but that would mean more such notifications for Nicholson and his marine successors. As the meeting continued, it seemed to Weston that Miliband was discreetly asking Nicholson to help Britain instead of raising the matter in a higher-profile way with Hillary Clinton or US defence secretary Robert Gates. Soon thereafter, Weston drafted a cable for Ambassador Eikenberry titled "US-UK at a crossroads". It argued that the United States needed to ease its closest ally out of the toughest parts of Helmand, not force the British to stick it out. Eikenberry agreed, and drafted a memo that he sent directly to Clinton.

Nicholson agreed to take on Musa Qala, but he had no desire for Sangin. Officers on his staff scoffed at the Brits. The point of the American surge, as they saw it, was to move into places where there were no coalition forces, not rescue the country occupied by the second-largest military contingent in Afghanistan.

British officers eventually decided to let the marines, who always seemed to be spoiling for a fight, find their own way to Sangin. It happened early in the summer of 2010. After the transfer in Musa Qala, the top British commander in Helmand had agreed to place his troops in Sangin under the control of the marines. Colonel Paul Kennedy, the top marine field commander in northern Helmand, quickly concluded that he needed more forces there. Soon after the new battalion arrived in July, British officials informed the Americans that the British unit in Sangin would not be replaced when its tour finished early that October.

By concentrating in and around Lashkar Gah and the districts of Gereshk and Nad Ali, the British military would finally have enough combat power to perform a genuine protect-the-population counterinsurgency mission. But before long, both sides resumed bickering. This time, Sangin replaced Musa Qala as the principal point of friction. The marines felt the British-run reconstruction office began paying less attention to Sangin once British forces departed. British officials denied the allegation, but they shouldn't have. Although the marines believed Sangin was the most critical part of the province – because the fighting was so intense – it wasn't. Lashkar Gah and Gereshk were far more populous and vital.

But the marines weren't all wrong: if they were going to succeed where the Brits had failed in Sangin, they required more than rifles and missiles; they needed meaningful resources for reconstruction.

The dispute once again illuminated how the two closest allies in Afghanistan failed to understand each other. Had the British not torpedoed their relationship with the marines through unseemly deals and a 19th-century attitude toward the Afghans, and had the marines not always equated British restraint with appeasement, the two militaries could have been true allies. The British could have emulated the American model of massing combat power to strike hard and fast. And the marines could have grasped the British wisdom in picking only the most important fights. The result almost certainly would have been fewer body bags draped with the union flag or the Stars and Stripes.