If there is one lesson that should be learned from more than a decade of combating the Islamic State and its previous incarnations, it is that no amount of foreign force can defeat the organisation without enlisting the help of an armed local resistance. Its foremost strategy is built around the objective of subduing locals and leaving them with no viable alternatives. It has largely succeeded in doing so in Iraq and, as the world fails to grasp this reality, it is making headway in much of Syria.

As Britain reflects on the wisdom of its decision to participate in the US-led campaign to “degrade and ultimately defeat” Isis in Syria, that formula should be the premise of any such move. And the UK has much to offer in this regard.

In 2006, the United States failed to weed out the group in Anbar — then known as the Islamic State in Iraq — until brigade commanders figured out a way to turn the population against it by working with an already existing tribal rebellion in Ramadi. The circumstances at the time were even more favourable than today: the US had a massive military presence in the country and the sectarian nature of politics in Baghdad was less pronounced.

Today, unintended consequences caused by the current campaign are even driving locals closer to the organisation or at least making them indifferent towards it. The US has thus far focused on working with particular forces in limited areas to battle Isis, while persistently striking the group’s bases and economic routes inside its heartlands. The tagetting of bridges and oil facilities and trucks is paralysing the economy in Isis-controlled areas. This sometimes pushed people to join the only employer in town to generate income for their families. Others have emptied their household of young members by sending them overseas as refugees.

Meanwhile, Isis embeds in residential areas to evade the airstrikes while still making money through taxation, extortion and other means that enabled it to take most of the areas now under control before it laid its hands on the oil infrastructure. It is also quietly expanding in less strategic but vulnerable areas such as the areas between Palmyra, the city of Homs and southern Syria, to avoid intensive bombardment or heavy military deployment.

Britain should not exacerbate the situation by merely deploying jets to fly more sorties onto Isis areas. Instead, most of the time and effort should be used to encourage and prop up local forces to fight ISIS. That requires a strategy that is independent from the one currently led by Washington. The focus for the UK should be to work mostly in the background through existing and new channels to advise, network, train and provide non-military services to armed fighting groups in different parts of the country.

For example, the UK has until recently sponsored an ambitious and unique programme to appoint moderate imams in an area controlled by various rebel forces, instead of extremist clerics affiliated to jihadi organisations. Part of the moderate clerics’ focus was to educate worshippers about the danger of takfir — or pronouncing fellow Muslims as infidels or apostates. According to a field commander of the faction overseeing the programme, the “culture of takfir” is a major impediment to getting fighters to combat groups such as Isis, especially if the faction is backed by western countries.

The UK also sponsors other important programmes that are not publicly discussed. One is a £10m programme to train a local police force in northern Syria. It is also involved in enabling rebel forces to provide relief work for communities living under them. There is even a programme to support anti-Assad Alawite activists living in regime-held areas. Some of these programmes tend to be short-lived and require increased and consistent funding.

Such programmes should be highlighted to the British public and expanded to other areas, targetting fighters driven out of Isis areas. These fighters have a proven record of fighting Isis and resisted it to the bitter end. Instead of abandoning the fight, however, they moved to southern and northern Syria to continue the fight against the organisation — with an eye to return to their areas when the time is right. Having seen how Isis controls an area, leaders of these factions say relief work and targeted programmes are key to fostering popular support to make up for the dwindling numbers of fighters as time goes by.

The UK has enough good will among the opposition. It could use its leverage and tap into such programmes to help fighters gain the local support needed to defeat Isis. Equally important, the UK should make it loud and clear that Bashar al-Assad has no future in Syria if it seeks to win the support of those most capable of defeating the group, while supporting the political process based on Geneva communique and Vienna talks.

The rebels, not the regime, operate in or near Isis-held areas. There are enough rebel forces with a proven track record of fighting the group. They are the key to defeating it.