On July 28 Austria declared war against Serbia, either because it felt Russia would not actually fight for Serbia, or because it was prepared to risk a general European conflict in order to put an end to the Greater Serbia movement. Russia responded by partially mobilizing against Austria. Germany warned Russia that continued mobilization would entail war with Germany, and it made Austria agree to discuss with Russia possible modification of the ultimatum to Serbia. Germany insisted, however, that Russia immediately demobilize. Russia declined to do so, and on August 1 Germany declared war on Russia.

The French began to mobilize on the same day; on August 2 German troops traversed Luxembourg and on August 3 Germany declared war on France. On August 2 the German government informed the government of Belgium of its intention to march on France through Belgium in order, as it claimed, to forestall an attack on Germany by French troops marching through Belgium. The Belgian government refused to permit the passage of German troops and called on the signatories of the Treaty of 1839, which guaranteed the neutrality of Belgium in case of a conflict in which Great Britain, France, and Germany were involved, to observe their guarantee. Great Britain, one of the signatories, on August 4 sent an ultimatum to Germany demanding that Belgian neutrality be respected; when Germany refused, Britain declared war on it the same day. Italy remained neutral until May 23, 1915, when, to satisfy its claims against Austria, it broke with the Triple Alliance and declared war on Austria-Hungary. In September 1914 Allied unity was made stronger by the Pact of London, signed by France, Great Britain, and Russia. As the war progressed, other countries, including Turkey, Japan, the United States, and other nations of the western hemisphere, were drawn into the conflict. Japan, which had made an alliance with Great Britain in 1902, declared war on Germany on August 23, 1914. The United States declared war on Germany on April 6, 1917.