German military planners had faced one basic strategic problem since the days of Frederick the Great. Squeezed between a hostile France in the west and a hostile Russia in the east (usually joined by a hostile Austria in the south), their only hope of avoiding defeat had always been to overwhelm one of their enemies before the other was in a position to intervene. Prussian victories in 1866 and 1870 had been made possible by Bismarck’s success in neutralizing Russia in both conflicts, but in 1891 the Franco-Russian Alliance had revived the dilemma in its starkest form. Which enemy should be destroyed first? Schlieffen had firmly settled for France. No decisive victory was possible in the huge plains of Poland, but, if France could be defeated, the Russians might quickly be brought to terms. But how to gain a rapid and decisive victory over France?
Since 1871 France had built such formidable fortifications along her German frontier that a repeat of 1870 appeared impossible. The only answer seemed to lie in an outflanking movement through neutral Belgium, one powerful enough to defeat the French army in time to switch forces eastwards to ward off the expected Russian assault. Schlieffen himself, as we have seen, did not take the Russian threat very seriously, but by 1914 it appeared such a menace that German planners sometimes feared that Russian armies might enter Berlin before their own forces had reached Paris. A massive invasion through Belgium was thus an essential part of German war plans, and the increase in the size of the German army resulting from the reforms of 1912–13 had been largely devised to make this possible.
Clausewitz once wrote that military plans might have their own grammar but they had no inherent logic. There was certainly no logic in the decision by the German General Staff that, in order to support the Austrians in a conflict with Russia over Serbia, Germany should attack France, who was not party to the quarrel, and do so by invading Belgium, whose neutral status had been guaranteed by a treaty of 1831 to which both Germany and Britain had been signatories. It was significant of the state of affairs in Berlin that the German Chancellor, Theodore von Bethmann Hollweg, saw it as his task, not to query this decision, but to justify it as a necessary breach of international law in the prosecution of a just and defensive war. But, in order for the war to appear just and defensive, Russia must be made to appear the aggressor, and this was the major concern of the German government in the last days of the crisis.
Serbia predictably rejected the Austrian ultimatum, and Austria declared war on 28 July. Thereafter military calculations dominated decision making in every European capital. On 30 July Czar Nicholas II, with extreme hesitation, ordered the mobilization of all Russian armed forces. It was generally assumed that mobilization led inevitably to Aufmarsch, the deployment of armies for the invasion of their neighbours, and that such deployment led with equal inevitability to war. Mobilization was thus like drawing a gun; whoever did so first enjoyed a huge strategic advantage. But, if Russia did not do so first, her administrative backwardness and the vast distances her reservists had to travel would put her at an equally huge disadvantage with respect to the more compact and better-organized Germany. In fact, neither for her nor for her French ally did mobilization necessarily mean war, but for Germany mobilization did lead seamlessly into Aufmarsch, and Aufmarsch into an invasion of Belgium time-tabled to the last minute. Russian mobilization gave her the excuse. Last-minute attempts by a panic-stricken Kaiser to delay matters were useless. The order to mobilize was given in Berlin on 1 August. An ultimatum demanding free passage through Belgium was issued the following day, and when it was rejected German troops crossed the frontier on 3 August.
In Britain the invasion of Belgium united what had until then been a deeply divided public opinion. Ever since the sixteenth century it had been an article of faith in British naval policy that the Low Countries should not be allowed to fall into hostile hands, and this belief had become almost visceral, irrespective of party politics. The British government at once issued an ultimatum demanding assurances that Belgian neutrality would be respected. It remained unanswered, and Britain declared war on Germany on 4 August. Liberal concerns for the rights of small nations combined with traditional conservative concern for the maintenance of the balance of European power to make parliamentary support almost unanimous. A state of war was proclaimed throughout the British Empire and the ‘First World War’ began.