A sudden surge of population puts too much strain on the social and physical infrastructure of receiving countries, but it also severely diminishes the hope that the forsaken country can ever recover.
Syria is leaking educated, qualified people. It has been evident in news coverage of the exodus that the migrants are mostly young, Westernised and enterprising. These are people no country can afford to lose but especially one under attack from backward religious and tribal forces, one of which wants to re-establish a medieval theocracy.
The regime of Bashar al-Assad, still determined to stay in power, presents at least as great a threat. Many people now on Balkan roads are fleeing from rebel cities suffering retaliation from the regime. It is Assad they fear, more than Isis.
Yet leaders gathering in New York this week face the unpleasant fact that if they are going to tackle the migration crisis at its source, they have to deal with Assad.
If they must choose between him and Isis, he at least seems to pose no threat beyond Syria's borders. But it is hard to stomach an autocrat who cares so little for the country that he would destroy it rather than give way to popular uprising as other military rulers did in that brief "Arab spring".
Only Russia's President, Vladimir Putin, appears comfortable supporting Assad, which he has done since the war began. President Obama and British Prime Minister David Cameron are contemplating the prospect through gritted teeth. New Zealand, says John Key, wants a deal with Assad that would include Assad's departure.
They have left their homes and their country because they have lost hope that the civil war might end any time soon. World leaders have to do something to rekindle that hope. If it means an agreement that leaves Assad in charge of the part of Syria he still controls, while international forces rid the country of Isis, it would seem to be worth the price.
If Western powers are drawn yet again into the tensions of a region they barely understand, so be it. A mass movement of humanity has put its problems on everyone's plate.

