War in Europe had been feared since before the turn of the century. Of course no one could predict what year it would start or how long it would last or if it could be avoided even.
Russell's original "time of tribulation" was from 1874 plus 40. He was aware of the political climate around the world and it surely influenced his belief in the "end times" scenerio. So if the war started anytime in that period, they could eventually twist their doctrine to include the events as momentous and prophecy filling.
One thing I've always found interesting is that the WTS loves to point to 1914 as the start of the war...and it is, but the US, home of the WTS/JWs didn't declare war until 1917. Just a minor thing, but when compared to WWII, most Americans talk about 1941 thru 1945 as WWII - the time of US involvement. But since 1914 fit so well with the WT theology, it was better to keep to the European timeline instead of the US one when talking about WWI.