In the trenches, infantry with gas masks.

The strategy of the Central Powers suffered from miscommunication. Germany had promised to support Austria-Hungary’s invasion of Serbia, but interpretations of what this meant differed. Austro-Hungarian leaders believed Germany would cover its northern flank against Russia. Germany, however, envisioned Austria-Hungary directing the majority of its troops against Russia, while Germany dealt with France. This confusion forced the Austro-Hungarian Army to divide its forces between the Russian and Serbian fronts.

Some of the first clashes of the war involved British, French and German colonial forces in Africa. On 7 August, French and British troops invaded the German protectorate of Togoland. On 10 August German forces in South-West Africa attacked South Africa; sporadic and fierce fighting continued for the remainder of the war.

The Serbian army fought the Battle of Cer against the invading Austrians, beginning on 12 August, occupying defensive positions on the south side of the Drina and Sava rivers. Over the next two weeks Austrian attacks were thrown back with heavy losses, which marked the first major Allied victory of the war and dashed Austrian hopes of a swift victory. As a result, Austria had to keep sizable forces on the Serbian front, weakening their efforts against Russia. Serbian troops then defeated Austro-Hungarian forces at the Battle of Kolubara, leading to 240,000 Austro-Hungarian casualties. The Serbian Army lost 170,000 troops.

Initially, the Germans had great success in the Battle of the Frontiers (14 August – 24 August). Russia, however, attacked in East Prussia and diverted German forces intended for the Western Front. Germany defeated Russia in a series of battles collectively known as the First Battle of Tannenberg (17 August – 2 September), but this diversion exacerbated problems of insufficient speed of advance from rail-heads not foreseen by the German General Staff. Originally, the Schlieffen Plan called for the right flank of the German advance to pass to the west of Paris. However, the capacity and low speed of horse-drawn transport hampered the German supply train, allowing French and British forces to finally halt the German advance east of Paris at the First Battle of the Marne (5 September – 12 September). The Central Powers were thereby denied a quick victory and forced to fight a war on two fronts. The German army had fought its way into a good defensive position inside France and had permanently incapacitated 230,000 more French and British troops than it had lost itself. Despite this, communications problems and questionable command decisions cost Germany the chance for an early victory.

New Zealand occupied German Samoa (later Western Samoa) on 30 August. On 11 September the Australian Naval and Military Expeditionary Force landed on the island of Neu Pommern (later New Britain), which formed part of German New Guinea. Japan seized Germany’s Micronesian colonies and after the Battle of Tsingtao, the German coaling port of Qingdao, in the Chinese Shandong peninsula. Within a few months, the Allied forces had seized all the German territories in the Pacific.