Arthur M. Schlesinger, Jr., "Islamophobe"?

I'm reading the new biography of the American historian and just came across this passage. It's an entry from Schlesinger's journal, written in May, 1940 when he was aged 22:

The central fact of the year is the war, which has changed our world for us...The invasion of Holland and Belgium finally awoke me to Nazism. Hitler is no mere imperialist conqueror, somewhat nastier and gaudier than the Kaiser...His [is] not a war for markets and colonies. It is a revolution and a crusade. The analogy is not the first world war. It is the spread of Mohammedanism. Hitler will not be controlled by the motives which controlled Cecil Rhodes, say, or even Napoleon. He is the prophet of a new religion, and like all prophets who believe their faith with sufficient intensity, his is out to convert or destroy.

How dare he call Mohammedanism destructive. Didn't he know it's the religion of peace?