Here's where ISIS got its weapons - and why it's so hard to stop them from getting more

ISIS has anywhere from 30,000 to 100,000 fighters and is one of the world's most dangerous terrorist groups.

But how did a group barely anyone knew of just a few years ago take over vast territories in Iraq and Syria and carry out terror attacks in Egypt and France?

Guns - lots of them. And how they got them is a murky tale of failed Soviet states, Middle Eastern gun culture, arms smuggling, and battlefield success.