The signs are unmistakable. Once again, the west is preparing to escalate military intervention in the Arab and Muslim world. This time the target is Syria. Since the US presidential election, the warnings have multiplied. First, in a breathtaking reprise of the falsehood that paved the way for the invasion of Iraq, US and British leaders claimed the Syrian regime might be about to use chemical weapons against rebel forces, and threatened dire consequences.

Then the US authorised the stationing of Patriot missile batteries along the Turkish-Syrian border. Ostensibly intended to protect Turkey from stray Syrian artillery fire, they could rather more plausibly be used to help enforce a Libya-style no-fly zone. There has since been a flurry of media briefings about increased covert US arms supplies and rebel training, along with plans for intensified intelligence and special forces deployment, or even all-out air and naval power support. Direct intervention, US and British officials are reported to insist, is "now inevitable".

Next Britain followed France in recognising the new opposition Syrian National Coalition, stitched together under Nato and Gulf tutelage, as the "sole legitimate representative of the Syrian people". Since the coalition clearly isn't the sole representative of Syrians, the declaration (which goes beyond even what was said during the Libyan war) sets a precedent that is likely to come back to haunt them. But it was followed by only a slightly less sweeping statement from the US and around 100 allies.

What such support can mean on the ground is demonstrated in the latest real-life horror video circulating among Syrians. It shows two captured officers from President Assad's Alawite sect being beheaded with a machete in the street, apparently by western-backed Free Syrian Army rebels, one of them a child.

Of the tens of thousands who have died since last year's uprising morphed into armed revolt, the majority have certainly been killed by regime forces. But there's also no doubt that atrocities have been committed on a large scale by both sides. And they have mushroomed as jihadist groups have grown in importance and Iraq-style ethnic cleansing, kidnapping, revenge killings and sectarian attacks spread.

Rampant torture and summary executions by opposition as well as regime forces have been condemned by human rights organisations, along with widespread rebel conscription of child soldiers. Last week Channel 4 News uncovered evidence that more than 100 Alawite civilians killed in the town of Aqrab may have been massacred by rebel forces rather than, as initially reported, by government troops.

You might imagine the multiplication of such incidents and the advance of fundamentalist groups in Syria would give western governments reason to pause before bolstering their support for the rebels. But in fact that's exactly why they insist they need to step up their involvement.

David Cameron told parliament this week that there was now a "strategic imperative" to act because the Syrian war (which the west and its Gulf allies have been fuelling) is "empowering al-Qaida-linked extremists". There is an "opportunity", he says, for Britain, the US and autocratic regimes such as Saudi Arabia and Jordan to "shape" the Syrian opposition.

Of course, both the US and Britain have been funding, training and attempting to funnel Gulf arms through Turkey and Jordan to their favoured factions for some time. Now the Obama administration has branded a leading Syrian jihadist group a terrorist organisation, to Syrian opposition fury. The aim is intervention for influence, both before and after the expected fall of the Assad regime – dressed up, as in Libya, in the language of "protecting civilians".

It's all of a piece with the rebranded war on terror. In the wake of the disaster of the Iraq and Afghanistan wars, there were to be no more boots on the ground. Intervention would again take the form of "humanitarian" air campaigns, targeted drone attacks and a return to the proxy and covert wars of the past.

But as demonstrated by Nato's campaign in Libya – which helped boost the death toll by a factor of at least 10 and gave air cover to ethnic cleansing and indiscriminate killing – wars to "protect civilians" do nothing of the kind. Deeper western intervention in Syria will certainly escalate, not end, the killing, as well as taking Syria's future out of the hands of its own people.

What began in Syria nearly two years ago as a popular uprising, brutally repressed by the Assad regime, has since increasingly taken on the character of a sectarian conflict and regional proxy war, as Saudi Arabia and its western backers have seen the chance to bury Iran and Russia's main long-term Middle Eastern ally.

But the expectation that the government is about to fall is almost certainly premature. With neither side strong enough to prevail, the likelihood instead is that the country will go on bleeding, as external intervention deepens the conflict. Even if the regime were to implode or retreat to its strongholds, the civil war would very likely continue.

Which is why the only way out of an increasingly grim conflict is a negotiated settlement, with regional and international backing. This week, Syria's semi-detached vice-president, Farouk al-Sharaa (mooted as a possible transitional president), acknowledged that the Syrian army could not win the war, and called for a "historic settlement" and national unity government.

The western powers and Gulf regimes have so far underwritten the opposition resistance to negotiation. An attempt to sponsor a regional settlement by Egypt's president, Mohamed Morsi, in conjunction with Turkey, Iran and Saudi Arabia was scuppered by the Saudis. But in one form or another, negotiation will eventually have to take place.

Meanwhile, not only will more intervention by the western powers increase the death toll. It may not give them the control they crave either. Already the mainly Islamist rebel fighters are becoming more mistrustful of their foreign backers. Just as likely is that it will lay the ground for the kind of blowback that created al-Qaida in Afghanistan in the first place – and risk engulfing the region in a still more devastating conflict.

Seumas Milne's book, The Revenge of History: The Battle for the 21st Century, is available from guardianbookshop