Let us do a thought experiment. Imagine the UN did not vote to authorise the use of force in Libya in March. Nato did nothing; Colonel Muammer Gaddafi over-ran Benghazi; the US stood by; the Libyan opposition was reduced to sporadic uprisings, quickly crushed. The regimes in Yemen and Syria took note, and put down their own uprisings with greater vigour. The west let brutality and oppression triumph again in the Middle East.

This is the scenario many wise heads were effectively arguing for with their strong stands against intervention to stop Col Gaddafi. Over the months those analysts have reminded us of their views, calling Libya a quagmire. This week one of the leading proponents of that position, my friend and colleague Richard Haass, shifted gears – but only to remind us just how hard the road ahead in Libya is likely to be.

I do not know anyone, regardless of the side they took in the initial debate, who thinks this task will be easy; indeed, the battle against Col Gaddafi is not yet won. But not so fast. Before we focus on what must happen next, let us pause for a minute and reflect on that initial debate and the lessons to be learnt.

The first is that, against the sceptics, it clearly can be in the US and the west’s strategic interest to help social revolutions fighting for the values we espouse and proclaim. The strategic interest in helping the Libyan opposition came from supporting democracy and human rights, but also being seen to live up to those values by the 60 per cent majority of Middle Eastern populations who are under 30 and increasingly determined to hold their governments to account. This value-based argument was inextricable from the interest-based argument. So enough with the accusations of bleeding heart liberals seeking to intervene for strictly moral reasons.

We also now know how different intervention looks when we help forces who want to be helped. East Timor, Bosnia-Herzegovina, Kosovo, Libya – all cases where force evened out odds between a brutal government and a widespread and legitimate social or national movement. It is difficult to know when a state has failed in its responsibility to protect its people, particularly when secession is involved. This is why international authorisation is both required and difficult to obtain. But the contrast with Iraq and Afghanistan, where external invasion saw the US often labelled as an enemy, is enormous.

Another clear lesson: the depiction of America as “leading from behind” makes no sense. In a multi-power world with problems that are too great for any state to take on alone, effective leadership must come from the centre. Central players mobilise others and create the conditions and coalitions for action – just as President Barack Obama described America’s role in this conflict. In truth, US diplomacy has been adroit in enabling action from other powers in the region, and then knowing when to step out of the way.

That said, we must not focus just on states, because Libya also shows that social forces are increasingly powerful drivers of foreign policy. Those forces have now pushed both the west and Arab governments into taking a much harder line than simply geostrategic logic would dictate against Bashar al-Assad’s brutality in Syria, and even (albeit timidly) against torture and killings by the Bahraini government. Social movements are also beginning to reshape politics in Israel and India.

Looking forward, it is really not up to the west, much less the US, to plan Libya’s transition. It is a relief to see so many articles and statements reflecting lessons learnt from Iraq. But the Libyans are far ahead of where the US was when the initial fighting ended in Iraq. The National Transitional Council has a draft constitutional charter that is impressive in scope, aspirations and detail – including 37 articles on rights, freedoms and governance arrangements.

The sceptics’ response to all this, of course, is that it is too early to tell. In a year, or a decade, Libya could disintegrate into tribal conflict or Islamist insurgency, or split apart or lurch from one strongman to another. But the question for those who opposed the intervention is whether any of those things is worse than Col Gaddafi staying on by increasingly brutal means for many more years. Instability and worse would follow when he died, even had he orchestrated a transition.

The sceptics must now admit that the real choice in Libya was between temporary stability and the illusion of control, or fluidity and the ability to influence events driven by much larger forces. Welcome to the tough choices of foreign policy in the 21st century. Libya proves the west can make those choices wisely after all.

Anne-Marie Slaughter, a member of the Atlantic Council, is Dean of the Woodrow Wilson School of Public and International Affairs and the Bert G. Kerstetter ’66 University Professor of Politics and International Affairs at Princeton University. This article was originally published at FT.