The book you mention is not unchallenged because of the onesided position the author takes. And sources in wartime are always filled with propaganda.
Verdun was not a victory for Germany but an offensive that failed to reach the goal (Verdun) and was repelled. In the end, both sides have lost an equal number of soldiers and no advantage. Without intervention of the USA, Germany would perhaps been able to delay the end of the war until 1920 or such, but no chance for winning.
But I think it's not the right place to discuss this. WW1 was one big military mistake.
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