Complex sentence: He served under the Turks in the siege of Vienna when they were defeated it is alleged that he conceived the plan of marching on Constantinople to drive the Turks out of Europe, the western powers having promised him their moral support.
Split into simpler sentences: He served under the Turks in the siege of Vienna when they were defeated. It is alleged that he conceived the plan of marching on Constantinople to drive the Turks out of Europe, the western powers having promised him their moral support.