Gavrilo Princip's fault for assassinating Archduke Franz Ferdinand. Without that assassination, no World War I, dissolution of the Ottoman Empire, or the eventual establishment of Iraq as an independent state. And thus, no ISIS.
The fact that ISIS probably wouldn't exist without the assassination of the archduke is quite valid especially as everyone in the past century has come to agreement it was a major blunder and f**k up and the consequences are being reaped today. Another dark mark on Princip's legacy.