“Not again!” was the instinctive reaction of officials in the United Nations and in national governments early this year, when it became apparent that Haiti, the poorest country in the Western hemisphere, was sinking rapidly into chaos, and the idea of a new international intervention began to be discussed.

We had been there, and done that, 10 years before. In 1994, a multinational force entered Haiti, with the blessing of the U.N. Security Council, and restored the elected president, Jean-Bertrand Aristide to power. Six months later, the U.N. itself assumed responsibility for the mission. A real — but, with hindsight, too brief — effort was made to put the country back on its feet and enable it to maintain stability, notably by forming, equipping and training a professional police force.

Yet here we are again. By Feb. 28, when President Aristide left, the police force had disintegrated and the country was, to all intents and purposes, in the hands of armed thugs. The following day the Security Council again authorized intervention by U.N. member states, this time giving the U.N. itself only three months to take on the security burden.

A preliminary U.N. team is now in Haiti, assessing what needs to be done, and what form the future U.N. presence should take. We can be sure the task will be a complex one. The Security Council has promised humanitarian and economic assistance, as well as help in the areas of governance, human rights and the rule of law.

If anything, the situation looks even more daunting today than 10 years ago. Weapons have proliferated and drug trafficking has gained a foothold. Haitians are frustrated and disappointed with the international community as much as with their own leadership. The events of February have exacerbated the polarization, making it hard to form a new government that both opponents and supporters of Mr. Aristide will accept as legitimate.

Should we have learned by now that outsiders cannot solve Haiti’s problems? This, after all, is a country that has just celebrated the bicentenary of its self-emancipation from slavery and colonialism. For a time in the early 20th century it was a U.S. protectorate. Should it not now be left alone to sort itself out?

That proposition is attractive only in the abstract. Haiti clearly is unable to sort itself out, and the effect of leaving it alone would be continued or worsening chaos. Our globalized world cannot afford such a political vacuum, whether in the mountains of Afghanistan or on the very doorstep of the sole remaining superpower.

The spectacle of human misery is harder to ignore than it used to be, but the crucial difference from the past is that chaos can no longer be contained by frontiers. It tends to spread, whether in the form of refugee flows, terrorism, or illicit trafficking in drugs, weapons and even human beings. No one wants to intervene, but ultimately there is no choice.

As we do so, we have many lessons to learn from past attempts.

One is that no one organization, donor or international partner can go it alone. We in the U.N. must work closely with our colleagues in Caricom and the Organization of American States, adopting an integrated and common approach. They have played a leading role in the current crisis. And they will have to remain engaged in Haiti, as regional partners, long after the blue helmets have left. Haiti must not again be isolated in its own neighborhood as it was in the past.

Another lesson is the importance of dealing effectively, and in good time, with potential spoilers. Experience in other countries emerging from chaos and conflict has taught us that large numbers of alienated and armed individuals can only be kept out of trouble if, as well as being disarmed, they are given real opportunities and jobs in the civilian economy. Without economic growth, militias all too easily re-form, and the cycle of poverty, violence and instability starts again.

But the most important lesson is that there can be no quick exit. Haiti will need our resources and our support for a long time. The current crisis is at least as much the result of irresponsible behavior by the Haitian political class as of omissions or failures in previous international efforts. This means that true success will involve helping new and more responsible political groups to emerge — building on the role played in the crisis by civil society.

That cannot be done quickly. A long-term effort — 10 years or more — is needed to help rebuild the police and judiciary, as well as basic social services such as health care and education.

Too often, such crises command our attention only when they are at their most acute — when the television images are too startling, the violence too appalling and the suffering of millions of people too much to bear. In a country like Haiti, it is only by sustained engagement, with both government and civil society, that we can help to build the institutions which enable democracy to take root.

The stakes are high — above all for Haitians, but also for us. Getting it right this time means doing things differently. Above all, it means keeping international attention and resources engaged for the long haul.