There could no more dreadful idea than to pour more armaments into the sectarian war now consuming Syria. Yet that is precisely what Britain's coalition government wants to do. The foreign secretary, William Hague, seemed on Monday to parody his hero Pitt the Younger by demanding "how long must we go on allowing … ?" and "what we want to see is …". Who is this we? But even Pitt would never be so stupid as to declare war on Syria, which is the only morally sound outcome of Hague's rhetorical mission creep.

For two years pundits have proclaimed the imminent fall of Syria's President Bashar al-Assad. High on Arab spring, they declared he would fall from the logic of history. Or he would fall because western sanctions would bring him down. Or he would fall because the media, as in the novel Scoop, were with the rebels and had decided they would win.

Assad has not fallen. He is still there, locked in the lethal Muslim schism that resurfaced with the demise of the region's secularist dictators. These have now almost all gone: the shah in Iran, Najibullah in Afghanistan, Saddam in Iraq, Mubarak in Egypt, Gaddafi in Libya. They had faults in abundance, but they succeeded in suppressing religious discord, instilling rudimentary tolerance and keeping the region mostly in order. This was in the west's interest, and the rulers, like those in the Gulf, were supported accordingly.

Turning turtle and abetting their downfall may yet prove the most disastrous miscalculation of western diplomacy since the rise of fascism. Prior to the Iraq war, Saddam persecuted the Shias, but their shrines were safe and intermarriage was common. After the war, Sunni and Shia are torn asunder, with a death toll of ghastly proportions. Similar agony may soon be visited on the Afghans. Libya's Tripoli is more unstable now the west has toppled Gaddafi, its fundamentalist guerrillas spreading mayhem south across the Sahara to Algeria, Mali and Nigeria.

These upheavals might have occurred without western intervention. The revolutions in Tunisia and Egypt were largely self-starting. Islamist parties often came to power, because they offered an alternative discipline to the existing regimes. But the west's sudden zest for "wars of choice", its meddling in the politics of Pakistan and its sabre-rattling in Iran have created a cause on to which neoconservative Islamism could fasten.

Al-Qaida was in 2000 a tiny group of fanatics. America and Britain have portrayed it as an all-powerful enemy, apparently lurking in support of every anti-secularist rebellion. Cameron calls it "an existential terrorist threat… to inflict the biggest possible amount of damage to our interests and way of life". Yet stabbings and bombings do not constitute an "existential threat". The UK is a stronger culture than Cameron appears to believe. There is no threat to its existence, while the chief damage being done to its way of life comes from the incompetence of its government.

Syria is at present certainly a claim on the world's humanitarian resources, to be honoured by supporting the refugee camps and aid agencies active in the area. Assad's suppression of revolt has been appallingly brutal, but he was Britain's friend, as was Saddam, long after his regime began its brutality. That is how things are in this part of the world. The west cannot stop them. To conclude that "we cannot allow this to happen" assumes a potency over other people's affairs that "we" do not possess.

Pouring arms into Syria will no more topple Assad or "drive him to the negotiating table" than did two years of blood-curdling sanctions. Hague knows this perfectly well, as he knows there is no way arms can be sent to "good" rebels and not to bad ones. He knows that if you want one side to win a civil war, the only honest way is to fight on its side. We did that in Kosovo and Libya. In Syria, Hague has fallen back on Kipling's "killing Kruger with your mouth".

The differences between Sunni and Shia, now tearing at nations in the Middle East, are deeply embedded in Islam. As the scholar Malise Ruthven has pointed out, outsiders preaching tolerance are no use. These disputes are intractable "since the acceptance of pluralism relativises truth". For Sunni to accept Shia and vice versa is for each to deny the faith.

Christianity, after centuries of similar bloodshed, has learned religious tolerance (though in Northern Ireland, Britain can hardly talk). Much of Islam has not. The one antidote lay in the rise of secular politics. This is the politics that Britain destroyed in Iraq and Libya, in the belief that it was bringing democracy and peace. It has brought chaos.

Britain's military judgment is no more coherent than its political. It thinks it can conquer Syria – which is what toppling Assad would require – by proxy. But sending weapons cannot make a difference, and will merely entice Britain into promising troops, unless it wishes to desert the rebels. Like American backing for the Taliban in the 1990s, the idea that "my enemy's enemy must be my friend" could yet see British special forces fighting alongside al-Qaida in Syria.

War holds a terrible appeal for democratic leaders. Most of Europe's rulers have other matters on their hands, but Britain and France, two nations whose ancient empires carved up the Levant between them, cannot keep out of it. They see national interest and danger where none exists. They cannot relieve Syria's agony, yet hope some vague belligerence might bring relief.

The reality is they hope that belligerence might draw attention from political troubles back home. That is the worst reason for going to war.