Uncertainty over Islamic State numbers clouds threat analysis

Hours before US President Barack Obama announced a new military offensive against Islamic State, one of his top counterterrorism officials testified to Congress that the al-Qaeda offshoot had an estimated 10,000 fighters.

The next day a new assessment arrived from the CIA: the terrorist organisation's ranks had more than doubled in recent months, surging to somewhere between 20,000 and 31,500 fighters across Iraq and Syria.

The enormous discrepancy reflects, in part, significant uncertainty among US intelligence agencies over the dimensions and danger posed by America's latest Islamist adversary.

But the trajectory of those numbers also helps to explain Obama's decision to go to war against an Islamist group that has yet to be linked to any plot against the United States.

In his speech, Obama laid out a rationale that leaned heavily on what-ifs. The United States had "not yet detected specific plotting against our homeland", Obama said. But Islamic State leaders "have threatened America and our allies", he said, and were on a path to deliver on those threats "if left unchecked".

The emphasis on hypotheticals was notable for a commander-in-chief who presided over the creation of a counterterrorism doctrine in which US strikes are supposed to be contemplated only in cases of imminent threat of violent attack. Faced with a terrorist group that is expanding faster than US spy agencies can chart it, the "imminent" threshold appears to have been set aside.

When asked about the revised estimates of Islamic State fighters on Friday, White House press secretary Josh Earnest said it indicated "that the group has had some recruitment success after the battlefield advances that they demonstrated back in June, and it reflects some better insight that the intelligence community has been able to gain into the activities" of Islamic State.