Business Insider spoke with Tim Marshall, author of "Worth Dying For: The Power and Politics of Flags" about the impact of the black flag of ISIS.

He said: "The ISIS flag is a pretty clever piece of marketing. The moment you see it, you see in conjunction with it the terror it has brought and the incredibly barbaric acts that have been done in its name and under that flag."

"It is synonymous with terror. It's also a very clever flag: it's square, which is unusual. Muhammad's flag was supposed to be squared, it's thought."

"The writing on it is the "shahada," the profession of faith, and also there's the seal of Muhammad. The writing is very ragged, very sixth century, very true to the original Islam, which of course is their message."

"They are saying: "We are not one of these modern nation states with all their gloss. We are back to

the original, so follow us." It's a very powerful symbol that terrorises one audience and attracts another."

"We need to understand the ISIS flag on many levels. In the "hadiths," the sayings of Muhammad, it is said that towards the end of days, which they are working towards, the black flags will appear in Dabiq (Syria). You have to crawl over ice to get to them, if necessary."

"Can you imagine the psychological impact on a true believer, of a jihadist? Who actually thinks to themselves: "We

have held ground and created a caliphate, more than Al Qaeda ever did, and we are fulfilling the prophecies." It will fill their belief in what they are doing."

Produced and filmed by Claudia Romeo. Specials thanks to Joe Daunt.