Initially dismissed by US President Barack Obama, along with other fledgling terrorist groups, as a “jayvee squad” compared to al-Qaeda, the Islamic State of Iraq and Syria (ISIS) has shocked the world by conquering massive territories in both countries and promising to create a vast new Muslim caliphate that observes the strict dictates of Sharia law.

In ISIS: Inside the Army of Terror, American journalist Michael Weiss and Syrian analyst Hassan Hassan explain how these violent extremists evolved from a nearly defeated Iraqi insurgent group into a jihadi army of international volunteers who behead Western hostages in slickly produced videos and have conquered territory equal to the size of Great Britain. Beginning with the early days of Abu Musab al-Zarqawi, the founder of ISIS’s first incarnation as “al-Qaeda in Iraq,” Weiss and Hassan explain who the key players are—from their elusive leader Abu Bakr al-Baghdadi to the former Saddam Baathists in their ranks—where they come from, how the movement has attracted both local and global support, and where their financing comes from.