The so-called Islamic State has repeatedly shown its ability to strike well outside the territory it controls.
An attack on an industrial plant near Baghdad yesterday was spectacular and deadly.
But it is their propensity to use chemical weapons that has caused most alarm among those fighting them.
For the Kurds the spectre of gas attacks is especially potent – after thousands were gassed by Saddam Hussein in Halabja in 1988.
Now the Kurds are facing a constant barrage of missiles – wondering which ones contain more than just explosives.
Hamish de Bretton-Gordon is an expert of chemical weapons and works for a company that makes gas masks.
He has just returned from Iraq, handing out masks and training the Kurdish fighters in how to use them.
While he was there, he saw first-hand the aftermath of chlorine attacks.
His call for more protective equipment for the Pershmerga hasn’t fallen on deaf ears.
The Defence Secretary Michael Fallon is considering helping them acquire more gas masks.
At present they only have a few hundred for a frontline force of 44,000.
But the Americans are warning the fight to liberate Mosul won’t be quick -even if the Pershmerga are better equipped.
They are unlikely to retake the mainly Arab city – that onerous duty is likely to fall to the Iraqi army.
And it is a long way from being willing and able to enter the city, with the heavy death toll that may entail.