Take a pleasant farm and field. Introduce a flow of water from diverse sources. If the water mixes into the soil and feeds the mix of crops, the result is greater prosperity. But if the water pools in stagnant, non-circulating ponds it feeds disease and pests. If travelers going back and forth between the farm and foreign places bring back weeds that take root in the ponds and spread, the entire farm can be ruined.

Something like this has been happening in Europe over the last few decades. From 1970, Europe – which had for four centuries been a vigorous exporter of people around the world – became an importer of people from Asia, Africa, and the Middle East. Immigration has generally been healthy for societies: new ambitious people entered America, Australia, Argentina, and Canada and created a quilt-work of different faiths and ethnicities. But in each of these cases, the immigrants were encouraged (or demanded) to assimilate. They were asked to learn the language, conform to the laws, attend the schools, and work alongside prior generations of immigrants, natives and mixed native-immigrant communities. They might keep their own weekend schools to transmit their historic cultures, cluster in certain neighborhoods, and enrich the local social scene with their own ethnic and religious festivals (what could be more American than St. Patrick’s Day?). But they aimed above all to succeed in the greater society, and their success was welcomed if they made the grade.

Like Frank Sinatra – the child of Italian immigrants from New Jersey – who told us that if you can make it in New York you can make it anywhere, immigrants aimed to blend in and take leading roles in entertainment, by founding their own businesses, and in the professions (breaking down the barriers to high-level corporate offices took longer).

Yet most Europeans and European leaders never viewed the immigrant workers from Asia, Africa, and the Middle East as valuable newcomers who, if required to adopt the manners and values of Europeans, could enrich and fully join their societies. They preferred to think of them as “guest workers” who would do their tasks and then return home, or as “multi-cultural” adjuncts to mainstream European societies who would cluster in their own ponds and remain separate, following their own culture’s rules and dictates rather than giving them up in order to blend into their new European homes.

Allowed to fester in stagnant pools with weaker services and education and limited employment opportunities, and few links to the broader society, the immigrant communities became filled with resentment and anger, especially among the younger, native born generations. Neither immigrants with a strong work ethic grateful to leave the dangers of their homeland, nor true natives with all the opportunities and full embrace of the society of their birth, the 2nd and 3rd generations born into the segregated pools of immigrant communities were in a position of fraught identity conflicts and economic anxieties – ripe territory for the invasion of radical messages promoting anti-Western attitudes and siren calls to join the fight to redeem and counter historical and current humiliations.

These frustrations broke out in recurrent racial and immigrant riots in Europe, from Brixton in England to the banlieues of Paris. But now add to this existing mix a new flag-bearer in the Middle East, a dramatic, media-savvy, and powerful new organization at war with the West and urging loyal Islamic youth to join their battle both in the Middle East and in Europe. How could the rise of such a group – first al-Qaeda and then, even more powerful and successful the Islamic State (ISIS) – not increase the intensity of violence and conflict between Muslim youth and Europe?

When the Arab revolts broke out in the Middle East in 2010, Western nations had an opportunity to vigorously support moderate elements in the inevitable revolutionary struggle of moderates vs radicals, or to stand back and let the revolutionary dramas play themselves out.

Of course, it did not take much knowledge of revolutionary history to know the outcome of the latter course: revolutions typically descend into civil war and the triumph of authoritarian and radical regimes. Only in older societies, with some prior experience of democracy, and strong links to outside democratic powers, are revolutions against authoritarian rule likely to lead to democratic outcomes. From Libya to Syria to Yemen, these conditions were generally lacking in the Middle East and North Africa, so in the absence of strong external support for moderates and efforts to isolate and extinguish extreme radical groups, dangerous outcomes were likely.

Yet as revolutions descended into civil wars and extremist groups like al-Qaeda spread and morphed and multiplied, Western powers did little. Perhaps believing that Western interventions in Iraq and Afghanistan had led nowhere (in entirely different conditions, as in Afghanistan the West entered a state already at war with the Soviets, and in Iraq the West itself overturned a stable dictatorship), no powers were willing to stay involved in trying to shift the balance in the 2010-11 revolutions. Perhaps they also believed that jihadism and violence in the Middle East would stay in the Middle East, ignoring the ease with which radical ideology has spread across borders throughout history, or the potential for easy and powerful linkages between the radical jihadists of the Middle East and the pools of anxious and radical youths that had developed in Europe.

The horrors and foolishness of this approach are now evident in the blood on the streets of Paris, traceable directly to the influence of ISIS. It will be a long struggle to change this situation; we have likely set up a decade or more of danger and struggle in European capitals and several decades of struggle in the Middle East.

It is important to immediately work to overcome the isolation of immigrant communities in Europe. The deal that Europe offers should be the same offered to immigrant communities (including Europeans) when they settle in countries of immigration such as America or Canada – you – or your children – can obtain full membership in the society including citizenship provided you accept the values and practices of your new society; but if you are unwilling to accept them you should go home.

In the Middle East there must now be a struggle that transcends the Sunni-Shi’a conflicts that have dominated for centuries; instead the battle must be between the forces of civilized order and the forces of barbarous disorder. The leaders of Iran and Saudi Arabia need to make clear on which side they want to be counted, and only if it is the former should they be admitted to full trade and relations with the rest of the world.

Several decades from now, our children will wonder at how our generation could have been so blind to the risks of climate change and simply kept polluting the skies. They will also wonder how our generation could have been so blind to the risks of unassimilated immigrant communities, and of conducting wars against jihadis in the Middle East with half-hearted and poorly planned overseas efforts combined with unchecked torture and surveillance at home.

It is not too late to change course, but the first step is to understand why the course taken in the past decades was so wrong.