Not just Christopher Clark, but Margaret McMillan and pretty well every other historian making yet another attempt to unscramble the causes of WWII. Although with differences of emphasis, there's pretty well general agreement that principal responsibility lies with neither Germany or Britain, but with the Austro-Hungarian Empire: which, alone of the Great Powers, was actively pursuing war throughout the complex diplomatic gaming of the previous decade. All the others, Germany included, were drawn in with varying degrees of reluctance.
‘WWII’
WW1 of course. Apologies
If I recall correctly, an American officer travelled through the capitals talking to his counterparts a year or two before WWI started. Regarding the gung-ho expectation of a quick short war he encountered but applying the American experience in the Civil War, he reported back that they were all crazy. The Europeans thought the high casualties of the American Civil War were due to incompetence.
However, the Imperial General Staff had warned the British government that any commitment of British forces to a European land conflict would destroy the entire 100K British Army in a few months. And so it was: by December 1914, almost the entire original force were casualties.