(CNN) Earlier this year I spent almost two months in northeast Syria covering what we thought was the final battle against ISIS. The so-called caliphate that stretched from the outskirts of Baghdad to western Syria, that ruled over more than ten million people, had been reduced to the remote town of Baghouz on the banks of the Euphrates.

In the end the "caliphate" was little more than a sprawling expanse of rubble and mangled cars, strewn with the corpses of ISIS fighters.

But it wasn't the final battle. We were wrong.

Thousands of ISIS fighters surrendered, and many thousands more -- their widows and orphans, their wives and children -- ended up in SDF-run prisons and detention camps

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Yet just days after the Turkish army and its Syrian allies invaded northeastern Syria last week, hundreds have escaped, and as the offensive continues and the SDF comes under ever greater pressure, more will probably break out of captivity.

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