The civil war in Syria has been raging for four years and its consequences have been nothing short of catastrophic: 250,000 Syrians have been killed, 12 million – or half the population – have become refugees and whole cities have been reduced to rubble. Islamic State controls a swath of territory almost the size of Britain, stretching from eastern Syria to western Iraq.

The plight of the Syrian refugees who recently reached Europe’s shores made this a burning issue in British politics. David Cameron, as is his wont, followed where he was supposed to lead. Public opinion shamed his government into doing something. Grudgingly, he announced that Britain would take in 20,000 Syrian refugees but only over a period of five years and the cost would come out of the overseas aid budget. Given the scale and urgency of the problem, this response is woefully inadequate.

Cameron seems to be itching to take the lead in preparing the ground for military intervention in Syria. He believes that the time is fast approaching for Britain to extend its air strikes against Isis targets from Iraq to Syria and he made this clear to the House of Commons when he said that “hard military force” would be necessary.

Downing Street is drawing up a new strategy for Syria that would involve limited military strikes against the “controlling brains” of Isis and a renewed diplomatic push to remove president Assad from power. Intent on avoiding another Commons defeat on Syria, Cameron has stressed that military action would be specifically directed at defeating Isis without elaborating on his plans for regime change in Damascus.

The new strategy is hopelessly muddled and has no chance of achieving either its declared or its undeclared objective. Air power alone cannot defeat Isis, as past experience demonstrates and as military experts acknowledge. But even if Isis is defeated, the result would be to strengthen rather than to weaken the Assad regime. The struggle for Syria is a three-cornered one between the Assad regime, Isis and the “moderate” Syrian rebels who fight both Assad and Isis. By concentrating firepower on Isis, Britain would relieve the military pressure on Assad and enable him to turn all his guns against the group the UK has been trying to foster, the “moderate” Syrian rebels.

British ministers keep repeating the mantra that Assad is part of the problem, not part of the solution. In truth he is a very large part of the problem but also an indispensable part of any negotiated solution. Policy has to be geared not towards wishful thinking but towards the reality on the ground and the reality is that all the efforts over the past four years to topple Assad have ended in failure. In other words, there is no military solution to the crisis in Syria. A political solution might be possible but only if Assad is involved in the talks on the future of Syria.

Here the lessons of history are highly instructive. As Churchill observed, the further back you go, the further forward you can see.

British ministers keep repeating the mantra that Assad is part of the problem, not part of the solution

Kofi Annan, on behalf of the UN, tried to broker a political compromise in 2012 that would have ended the fighting in Syria. Western insistence on regime change in Damascus sabotaged his efforts and forced him to resign. He was succeeded by Lakhdar Brahimi, the former Algerian foreign minister and one of the ablest Arab diplomats of his generation. Brahimi was armed with a double mandate to tackle the Syrian crisis, backed by both the UN and the Arab League. He convened an international conference in Geneva in January 2014. But on day one of the conference, the US secretary of state declared that Assad had to go. Not surprisingly, the Syrian government refused to enter into talks on the formation of a transitional authority and the conference got nowhere.

The only prospect of ending the horrendous death and destruction spawned by the Syrian civil war lies in convening an international conference under UN auspices with all the major parties to the conflict: the Syrian government, the “moderate” Syrian opposition, the US, Britain, France, Russia, Iraq, Iran, Qatar, Saudi Arabia and Turkey. The participation of Russia and Iran is particularly crucial because they have always supported the Assad regime and they are not going to abandon it any time soon.

The aim of the conference would be to strike a grand bargain. This would stipulate that the Assad regime could continue but only if it stopped the brutal attacks on its citizens and set in motion a political process leading to eventual power sharing. It would also unite most of the parties to this extraordinarily complex crisis against the common enemy, namely, Islamic State.

For Britain and its western allies to continue to insist that Assad must go would simply ensure that no negotiated settlement is ever reached. Politicians, like everyone else, are free to repeat the mistakes of the past, but it is not mandatory to do so.

Avi Shlaim is author of The Iron Wall: Israel and the Arab World