David Cameron made a reasonable case last week for Britain going to war with Isis in Syria; what he did not do is explain how this war is going to be won by Britain or anybody else. Even now, 18 months after Isis captured Mosul, there is a tendency by world leaders to underestimate its political and military strength.

Mr Cameron said that “military action [by the US, UK and others] seeks to degrade Isis’s capabilities, so that Iraqi security forces can effectively secure Iraq and moderate forces in Syria can defend the territory they control”.

It would certainly be nice if that happened, except that the Iraqi state security forces are demoralised, dysfunctional and have had difficulty finding new recruits since they have been repeatedly defeated by Isis over the past two years. In Syria, we are to look to 70,000 “moderate” fighters whose existence Mr Cameron revealed to the House of Commons, but nobody in Syria has ever heard of.

Isis is not going to be defeated by these phantom armies which are to be Britain’s allies in Iraq and Syria. It is the same weakness as in Iraq in 2003 and in Afghanistan a little later – in both cases, Britain was a junior partner in a US-led coalition that pretended to have local allies, but in practice these were too feeble to contribute much.

Does this matter much? After all, the British military contribution in Iraq has been small, some 360 air strikes out of a total of 5,432, and will be equally meagre in Syria. But this misses the point because, since the summer, Isis has extended its strategy of urban terrorism, so long used to slaughter non-Sunni Iraqis, to the rest of the world, as we have just seen in Ankara, Sharm el Sheikh, Beirut and Paris.

British Prime Minister David Cameron (AP)

Isis is well pleased by the results since it has succeeded in showing its power to the world, by all reports. It is more than likely that it will retaliate against British citizens in response to Britain’s joining the war in Syria. This consideration should not influence British decisions about what to do in Syria and Iraq, but such retaliation should not come as a surprise. Britain is still seen in the Middle East as a great power, whatever its actual status, and Isis is probably pleased that it has joined the roster of its enemies since it knows that British air strikes do not add much to the attack it is facing.

It is extraordinary how Isis has come to dictate the political agenda of a large part of the world. In Britain, its actions and British reaction to them have dominated the news since the killing of Mohammed Emwazi, nick-named “Jihadi John”, on 12 November. This is quite intentional on the part of IS, and one of the attractions of Paris as a target is that it is a news hub. The same would be true of London. In neither city is there any need for IS to make videos of atrocities; it can be sure of wall-to-wall coverage by cameras from every television company in the world. It will also be pleased in Raqqa by the near-hysterical tone of much of the reporting: bad as these mass murders are, they pose no existential threat to France, as the German invasions in 1914 and 1940 did.

The impact of Isis is measured not only in publicity, but in shaping relations between states. Poland and the Baltic states are alarmed to discover that the US and Western Europeans are becoming much more interested in finding a way of cooperating with Russia over Syria than in opposing its actions in Ukraine.

In pictures: Russian air strikes in Syria Show all 19 1 /19 In pictures: Russian air strikes in Syria In pictures: Russian air strikes in Syria Syrian boys cry following Russian air strikes on the rebel-held Fardous neighbourhood of the northern embattled Syrian city of Aleppo Getty In pictures: Russian air strikes in Syria Russian defense ministry spokesman Major General Igor Konashenkov speaks to the media in Moscow, Russia. Konashenkov strongly warned the United States against striking Syrian government forces and issued a thinly-veiled threat to use Russian air defense assets to protect them AP In pictures: Russian air strikes in Syria Syrians wait to receive treatment at a hospital following Russian air strikes on the rebel-held Fardous neighbourhood of the northern embattled Syrian city of Alepp Getty In pictures: Russian air strikes in Syria Russian Deputy Defense Minister Anatoly Antonov speaks at a briefing in the Defense Ministry in Moscow, Russia. Antonov said the Russian air strikes in Syria have killed about 35,000 militants, including about 2,700 residents of Russia AP In pictures: Russian air strikes in Syria Jameel Mustafa Habboush, receives oxygen from civil defence volunteers, known as the white helmets, as they rescue him from under the rubble of a building following Russian air strikes on the rebel-held Fardous neighbourhood of the northern embattled Syrian city of Aleppo Getty In pictures: Russian air strikes in Syria Civil defence members rest amidst rubble in a site hit by what activists said were airstrikes carried out by the Russian air force in the town of Douma, eastern Ghouta in Damascus, Syria Reuters In pictures: Russian air strikes in Syria A girl carrying a baby inspects damage in a site hit by what activists said were airstrikes carried out by the Russian air force in the town of Douma, eastern Ghouta in Damascus, Syria Reuters In pictures: Russian air strikes in Syria Civilians and civil defence members look for survivors at a site damaged after Russian air strikes on the Syrian rebel-held city of Idlib, Syria Reuters In pictures: Russian air strikes in Syria Civilians and civil defence members carry an injured woman on a stretcher at a site damaged after Russian air strikes on the Syrian rebel-held city of Idlib, Syria Reuters In pictures: Russian air strikes in Syria Volunteers from Syria Civil Defence, also known as the White Helmets, help civilians after Russia carried out its first airstrikes in Syria In pictures: Russian air strikes in Syria The aftermath of Russian airstrike in Talbiseh, Syria In pictures: Russian air strikes in Syria Smoke billows from buildings in Talbiseh, in Homs province, western Syria, after airstrikes by Russian warplanes AP In pictures: Russian air strikes in Syria Russian Air Forces carry out an air strike in the ISIS controlled Al-Raqqah Governorate. Russia's KAB-500s bombs completely destroy the Liwa al-Haqq command unit In pictures: Russian air strikes in Syria Caspian Flotilla of the Russian Navy firing Kalibr cruise missiles against remote Isis targets in Syria Â© TASS/ITAR-TASS Photo/Corbis In pictures: Russian air strikes in Syria Russia claimed it hit eight Isis targets, including a "terrorist HQ and co-ordination centre" that was completely destroyed In pictures: Russian air strikes in Syria A video grab taken from the footage made available on the Russian Defence Ministry's official website, purporting to show an airstrike in Syria In pictures: Russian air strikes in Syria A release from the Russian defence ministry purportedly showing targets in Syria being hit In pictures: Russian air strikes in Syria Russia launched air strikes in war-torn Syria, its first military engagement outside the former Soviet Union since the occupation of Afghanistan in 1979. Russian warplanes carried out strikes in three Syrian provinces along with regime aircraft as Putin seeks to steal US President Barack Obama's thunder by pushing a rival plan to defeat Isis militants in Syria In pictures: Russian air strikes in Syria Caspian Flotilla of the Russian Navy firing Kalibr cruise missiles against remote Isis targets in Syria, a thousand kilometres away. The targets include ammunition factories, ammunition and fuel depots, command centres, and training camps Â© TASS/ITAR-TASS Photo/Corbis

Whatever formal support there was in Nato for Turkey over the shooting down of a Russian aircraft, President Putin’s denunciation of Turkey’s support for extreme jihadis over the past three years has struck home. If there are more Isis mass killings of civilians from Western Europe or the US, pressure for a closer alliance with Russia is bound to grow.

The crisis is increasingly centred on a small part of the Syria where foreign powers and local proxies jostle each other. This is the north-west corner of Syria between the Euphrates and the Mediterranean, and south of the Syrian-Turkish border. It was here that the Russian plane was shot down by a Turkish jet on 19 November and where there has since been intense fighting. The Syrian army, backed by heavy Russian air strikes, has gained control of an important position known as the Turkmen mountain.

Some 10,000 Syrian soldiers are reportedly battling 6,000 Turkmen, the Army of Conquest, and the al-Qaeda linked Nusra Front (underlining the problem with David Cameron’s 70,000 moderate fighters: the Turkmen might be described as moderate, but they are fighting alongside the Syrian branch of al-Qaeda). The Syrian armed opposition has always depended on the border with Syria being open and, if the Syrian army and the Russians begin to close it, they will have gone a long way towards winning an important victory in the war. Half of the 550 mile-long frontier is already held by the Syrian Kurds.

Paris attacks: UK will help France in Isis fight, Cameron tells Hollande at meeting

These parochial struggles are having international implications. It was one of the greatest diplomatic miscalculations in recent history to imagine that the Syrian war would not affect neighbouring states. In the past six months alone, developments in northern Syria have led to a renewed conflict between the Turkish state and the Kurds and to confrontation between Turkey and Russia. Isis will be pleased by both.

Despite all the furious rhetoric after the Paris killings, Isis does not look as if is going to be under pressure that it cannot withstand. Both Russia and Russia’s critics have for different reasons portrayed Russia’s military intervention as being on a larger scale than it really is. It has fewer than 100 fixed-wing aircraft and helicopters operating in support of the Syrian army. This may be enough to reverse the advances by the Army of Conquest (mostly al-Nusra Front and Ahrar al-Sham) which, backed by Turkey, Saudi Arabia and Qatar happened in May. But it is not enough to give a depleted Syrian army the strength to win a decisive victory like the capture of the rebel-held half of Aleppo. The Iranian role also tends to be exaggerated, both by Iran and its enemies. The Pentagon says that there are fewer than 2,000 Iranian troops in Syria and 1,000 in Iraq.

The politics of war in Syria are extraordinarily complicated, and it was probably inevitable that Britain would be sucked into this morass. Everybody agrees that there is no appetite today in Britain for sending ground troops, though this could change overnight if there were a repeat of the Paris massacre in London.