On August 3, 1914, Belgium rejected a demand to allow the free crossing for the German Army leading to their invasion hours later. Germany declared war on France as well, beginning World War I. An excerpt from the article:
"After declaring war on France, Germany was now determined to execute its war plan to defeat France first and then concentrate its forces against Russia. The plan required German troops invade Belgium to get to France. This would be in direct violation of Belgium’s neutrality, which had been guaranteed in a treaty signed by major European powers, including Britain, in 1839. On the evening of 2 August 1914, Germany demanded that its troops be allowed to pass through Belgian territory. Belgium refused. Accepting Germany’s demands would make Belgium complicit in the attack on France and partially responsible for the violation of its own neutrality.
Germany’s invasion of Belgium tipped the balance for Britain. At 2pm on 4 August, it issued an ultimatum demanding Germany withdraw its troops. At 11pm, the deadline passed without a reply. Britain declared war. Britain’s entry into war was partially a reaction to larger anxieties about the balance of power in Europe, as well as its own security and position in the world. But by violating Belgium’s neutrality, Germany positioned itself as the belligerent aggressor and made British intervention a moral issue about the rights of small nations. The entry of Britain and its empire made this a truly global war. Europe’s leaders went to war with the general support of their citizens. This was especially important in Britain, where there was no compulsory military service and recruitment would be dependent on voluntary enlistment."