There are other flairs here who can likely comment better than I, but as I'm reading Christopher Clark's The Sleepwalkers right now some of this information is fresh. One of the reasons why "the inexorable slide into war" line is so common is that it was an almost universal feature in the memoirs of the principle players. Regardless of what country they were in, the leaders in 1914 tended to later write about the war in a way that, if it didn't quite strip the outbreak of war of all human agency, certainly seemed to make into something that passively occurred rather than that they themselves caused.

That style then permeates quite a lot of some of the most popular writing about the war, but has been pushed back against by later writers who have looked to balance inexorability with contingency with individual human agency with the unbelievably complex array of forces that led to the outbreak of war.

So one such example is that in German strategic thinking, they did not necessarily think that a general European war was inevitable, but they did think that if it was inevitable or was being forced upon them it would be better to do it now (in 1914) rather than later, before the Russians gained what they predicted would be an insurmountable military advantage. Perhaps paradoxically they also thought that if Russia declared war on Austria-Hungary for declaring on Serbia, despite the deterrent effect of Germany's present advantage, that would be proof of Russia's bellicosity, not Austria's or their own.

Many people in 1914 thought they could preserve the peace, but few realized that many of the systems that they had put in place to do so made war more likely, not less. So for instance, out of fear that Russia would back out of the entente, France had run through a number of Balkan crisis scenarios in which they basically made it clear to Russia that they would be a willing ally regardless of who was strictly responsible for conflict there. Elements in Austria-Hungary, in contrast, came to believe that a show of force in the Balkans would be the most likely way to force Russia to back down, as they had done in the past, and that a purely diplomatic approach wouldn't work. So both sides, in trying to preserve a system that they believed was vital for peace (the entente, and Balkan stability, respectively), had inadvertently made war more likely by failing to understand the systemic results of the other side's policies (moreover, these agreements were secret, so how could they?)

Historians generally are actually loathe to write about history as being inevitable, but the question for WWI seems to be when did it become inevitable, not whether it was inevitable generally. If I read Clark correctly he does not seem to think it became inevitable until the killing of Franz Ferdinand (which was itself a rather freakishly contingent event), and had the crisis passed probably would not have been possible later on given, for example, that the challenges to the alliance structures might well caused a major readjustment within only a few years.