Back in late August, the U.S. bombed a convoy of buses transporting ISIS fighters and their families who were fleeing Raqqa.  The convoy had been trapped at the Syria-Iraq border.  At the time, the U.S. said it would not allow the ISIS fighters to escape.

But political and strategic realities came into play when, as we discover today, Kurdish fighters who bore the brunt of the successful attack on Raqqa requested safe passage for the convoy across Syria. 

The U.S. did not intervene when hundreds of ISIS fighters and their families escaped – with their weapons.  Now Great Britain and other European countries are expressing alarm because the fighters are dispersing, and they believe that many of them will make their way home.

Daily Mail:

The exodus is understood to have been agreed to spare Kurdish forces, the BBC reported. They were leading the fight against IS on the ground and it would have required a fight to the death to clear the last surviving jihadis from the capital of their self-declared caliphate.

A truce was negotiated with local leaders to allow the remaining fighters and their families to leave. A Western coalition officer was present but not actively involved.

The convoy, also said to have been carrying tons of weapons and ammunition, left on October 12 bound for a camp further north in territory still held by IS. 

The coalition monitored the convoy from the air, dropping flares to help drivers in the 163 vehicles, including lorries and buses, navigate the road, witnesses told the BBC. 

IS fighters carrying AK47s sat on top of some of the trucks and squeezed into trailers.

The coalition forces said they did not condone the convoy but had been assured no foreign fighters would be on it and had believed only four had sneaked on, all of whom were in custody.

'We didn't want anyone to leave,' said Colonel Ryan Dillon, a spokesman for the coalition. 

'But this goes to the heart of our strategy – "by, with and through" local leaders on the ground.'

However, one driver hired to take part in the convoy said there was a 'huge number' of foreign fighters among the thousands involved, including many from Europe.

Another mentioned a number of foreigners – including female fighters.

Many of those who escaped have now moved on from Syria, with human traffickers on the Syria-Turkey border reporting a boom in business with the influx. One who has helped smuggle 20 families into Turkey in the past week said most were foreign. 'Some were talking in French, others in English, others in some foreign language,' he told the BBC.

Others went to the city of Idlib, to the west of Raqqa, where many Britons have also escaped from the country, usually paying £3,000 per fighter, it was reported.

So Raqqa was taken, but at what cost?  The Kurds don't care if the ISIS fighters migrate to Europe.  Certainly Bashar Assad and Hezb'allah don't care, either.  Iraq has enough problems with ISIS that it can't afford to care.

So that leaves the U.S. as the only member of the coalition against ISIS with a stake in where the fighters end up after they are defeated.  It is here that our complex relationship with the Kurds got in the way of the strategic necessity to kill as many ISIS fighters as possible.

Making concessions so that the Peshmerga did not have to fight a war of attrition against ISIS in Raqqa, saving their tough little army from being severely weakened, is ultimately in our long-term interest.  A strong Kurdish military, allied with the U.S., will be important as the crisis in the region unfolds over the next few months and years.

But it's still a bitter pill to swallow when you realize we had hundreds of ISIS fighters at our mercy, and they escaped to fight – and kill – another day.