There were many separate indicators that a pan-European war was in the offing but the realisation of inevitability could only be in retrospect when those indicators could be viewed collectively for their significance.
To get the debate going I believe that Bismarck’s astute assessment of Germany’s geographical position was prescient and enduring. The annexation of Schleswig enabled the extended Germany to dig the Kiel Canal across the promontory and provide a direct link between the Baltic and the North Sea for both commercial trade and for passage of the German High Seas Fleet into the open sea, almost in secret.
In 1893, France and Russia concluded an entente, almost an alliance, based upon their shared concerns over the growing power and belligerence of Germany. Britain though was still cool toward all three but mostly concerned by having to stretch its naval resources in the China Seas to oppose any Russian ambitions there. Britain proposed a mutual assistance pact with Germany but since that country could see little advantage in Britain’s naval power it declined. Remarkably, Britain became allied instead with Japan which was victorious in the Russo-Japanese War of 1904-1905 both at sea and on land.
In 1904 Britain and France concluded the Entente Cordiale which, in 1909, was extended to become the triple entente with Imperial Russia included.
In 1905 Kaiser Wilhelm II received his commissioned plan drawn up by Field Marshal Alfred von Schlieffen for a concerted though misconceived engagement against France, aimed at a 40 day strike, encircling Paris then meeting up with other German thrusts through Alsace Lorraine, corralling the entire French Army.
In 1906 Britain unveiled its new battleship, HMS Dreadnought, the revolutionary capital fighting ship, an all steel and iron construction with much larger primary armament. The project was of the highest priority to the First Sea Lord, Sir John (Jackie) Fisher, as essential to maintain the dominance of the Royal Navy. It made its impact to be sure but in making all other navies’ ships redundant it had done the same to its own! This realisation led directly to the ‘Great Naval Race’ between Britain and Germany. Bucking the current trend Britain had, by 1914, out-produced Germany by 50% due largely to Britain’s long coastline and multiple shipyards.
While diplomatic activity appeared to be aimed at defusing tensions in northern Europe, German foreign policy was, in fact, being set by a military elite rather than by Bethmann-Hollweg’s government. Events in the Balkan countries were moving inexorably towards a crisis which required only a marshalling of alliances to escalate into full scale war. The relationship between Germany and the Austro-Hungarian leadership was generating its own momentum in that direction. For this axis of Central Power purpose, the foresight of Bismarck in minimising Austrian alienation during the Austro-Prussian War was further testament to his judgement.
As There Was a Time explains in more detail, many of the alignments were already in place, notably Russia and Serbia on one side and Germany and Austria-Hungary on the other. Neither Britain nor France were fully prepared for the onslaught which quickly followed the progressive mobilisation of armies across the Continent. The descent into World War I quickly became as brutal as it was unstoppable.