Why not?
WWI was started by a Mohammedan.
You are mistaken. WWI was triggered by a Serb (at least notionally Orthodox Christian) nationalist attacking a member of a Roman Catholic dynasty. The only thing Mohammedan involved would, perhaps, be the plurality of religious identification in the region where the assassination took place.
(Of course, the Austrians didn’t need to make demands of the Serbs that they knew would be rejected as part of their just demand to bring the murderer and his confederates to justice simply to have a causus belli — Serbia agreed to all but one of the demands, but that wasn’t enough for the Austro-Hungarians.)