David Cameron’s latest comments on Iraq and Islamic State in Iraq and the Levant (Isis) terrorism bear all the hallmarks of a raging debate within the governing parties. He says the Isis threat means that “we need a firm security response, whether that is military action to go after the terrorists, international cooperation on intelligence and counter-terrorism, or uncompromising action against terrorists at home”. That may be what he wants, but it is not what the government has been delivering.

Britain has so far done little except tip food parcels out of military transport; the last week has been notable for dither and delay. There have been no British air strikes to support US disruption of the attacks on the Yazidis. Such is the shadow of the government’s parliamentary defeat on the Syrian intervention almost exactly a year ago.

The case on Syria was clear: Bashar al-Assad used chemical weapons to attack the dissidents contrary to international treaty. However, the vote was botched. MPs were called back from holiday grumbling that they could have waited another few days. The whips had no time to talk through the issues. Labour played hard-to-get, putting forward its own motion and ultimately refusing to back the government. The prime minister went down to a humiliating defeat. There were 30 Tory rebels and nine Liberal Democrats.

There is nothing more embarrassing than leading your troops over the top, only to discover that they stayed behind in the trench. Given that Cameron had been militantly urging President Obama to intervene, his embarrassment was complete: it was not merely a domestic inability to deliver, weakening the prime minister’s authority, but also a diplomatic disaster. In the wake of such a political car crash, following suddenly looks more attractive than leadership.

The prime minister represents both of the strands that former Labour MP David Marquand identified in the Tory tribe: the Whig imperialists who believe in muscular intervention in good international causes; and the Tory nationalists who want nothing if it does not serve narrowly defined British interests. To keep both on board, Cameron knows he has to travel slowly and build his coalition.

It is a debate that has raged throughout Tory history, most famously pitting Winston Churchill against Neville Chamberlain on appeasement. The most significant aspect of Cameron’s comments yesterday was his keenness to anticipate the Tory nationalist criticism of the past Syrian intervention.

Cameron argues that the Isis ambition for an Islamist caliphate, far from being a far-off conflict between people of whom we know little, represents a direct threat to British interests. This is the argument we heard about Afghanistan and Yemen, and there is some truth in it. Terrorist groups can strengthen and train under the protection of a sovereign state, as happened under Colonel Gaddafi in Libya, or in an area without effective state power, as in the Afghan Hindu Kush. And they will train young Britons.

In Iraq today, however, the urgency is humanitarian. We have a responsibility to protect people threatened by genocide, whether in Rwanda, Bosnia – or Iraq. That duty must be tempered by the capacity to act, and care for the consequences, but it is as much a part of our United Nations commitments as the universal declaration on human rights. It is not a counter-argument to say that we do not punish or correct other breaches of international law. Being incapable of always doing good is not a case against doing good where you can.

The Kurdish Yazidis are only the latest vulnerable minority to need protection. I vehemently opposed the Iraqi war in 2003 because the evidence for weapons of mass destruction was not there, and the UN weapons inspectors wanted more time to test the CIA and MI6 claims. But we do not need intelligence briefings to see what is going on in Iraq. The reporting of brave journalists provides photographic proof on every kitchen table.

This is crisis management, and it would be better if the world’s crises could be settled by an international body rather than relying on the United States and a “coalition of the willing”. But the UN has no strike force and no air power. It has no battalions save in peace-keeping missions when the shooting stops. Better Washington’s policeman, however occasionally self-serving, than no policeman at all. Better still a coalition of nations that underlines the international legalities. Even better, regional powers joining to meet their responsibilities. The United States alone still carries too much baggage.

That could also be the beginning of a longer solution. Europe’s luck is that borders no longer matter. The Middle East’s tragedy is that they still do.

The Frisians, the continental language group closest to English, live across Germany and the Netherlands, but who cares? They are allowed to speak their language. They can travel and worship freely. They have civil rights. The EU is a modern and civilised way of making sense of the diversity left behind by dynastic states.

The Middle East’s borders were largely drawn by the same negotiators at the same Paris Peace Conference in 1919, dismembering not just the Hapsburg but also the Ottoman empire, both traditionally happy to tolerate a minority patchwork settling throughout their lands. The 30 million Kurds were one of the biggest ethnic groups not to win their own state.

It took Europe another world war and the Holocaust to drive its competing nationalisms to accept the need for the supranational European Union. Even in the Middle East diversity does not need to be glued together by dictatorship. There is another option, as Lebanon showed before its destabilisation by the Palestine Liberation Organisation: you can have a functioning state that wins loyalty from different ethnic groups because of its tolerance and effectiveness.

Iraq has been unlucky. With Nouri al-Maliki gone, there is at least a chance to win back Sunni and Kurdish support for the Iraqi government. The new prime minister, Haider al-Abadi, is consensual not confrontational.

Imagination will be needed to make Iraq work. You would not bet your last penny on success, but it is the best chance Iraq – and perhaps the wider Middle East too – has. Cameron was thrown from his horse over Syria, but he now has to get back on and ride.