[nomadking]

Link.

Officials say the source for the 70,000 figure is the UK's Joint Intelligence Committee (JIC), drawing on assessments by the Foreign and Commonwealth Office, UK intelligence agencies and open sources.
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Charles Lister of the Brookings Institute puts the number of fighters in strategically influential groups at around 65,000 - with 10,000 fighters in smaller groups.

Regardless of the lack of clarity, the 70,000 figure does not include the Kurds.
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The SDF generally operates in a tense ceasefire with the Assad regime and has even been accused of being in open alliance with it, and as a result the British government does not include the YPG or the SDF - which may have as many as 40-50,000 troops - in its estimate.

So, counting the number of "allies" that the West considers vital to its dual strategy of defeating IS and pushing Mr Assad to the negotiating table makes the magical 70,000 number actually more like 100-120,000.