The Eastern Front and the Balkans played a significant role in shaping World War I. On the Eastern Front, the battle for supremacy between the Central Powers (Germany and Austria-Hungary) and the Allies (Russia, Serbia, and later Romania) led to large-scale military operations and decisive battles. One of the key battles was the Battle of Tannenberg in August 1914, where the German army successfully defeated the Russian forces and established dominance in the region. This victory allowed Germany to divert its attention to the Western Front and avoid a two-front war.

In the Balkans, the complex network of alliances and rivalries contributed to the outbreak of the war. The assassination of Archduke Franz Ferdinand, heir to the Austro-Hungarian throne, by a Serbian nationalist in June 1914, ignited a series of events that eventually led to the conflict. Austria-Hungary's subsequent ultimatum to Serbia, supported by Germany, triggered a chain reaction of mobilizations and declarations of war, dragging the Balkans into the broader conflict.

The Serbian campaign was a crucial part of the war in the Balkans. Austria-Hungary sought to punish Serbia for its alleged involvement in the assassination and regain control over the region. However, Serbia's resilient defense halted the initial Austro-Hungarian advances. The Serbian forces, supported by their strong sense of nationalism and guerrilla tactics, managed to hold off the invaders for much longer than anticipated. Eventually, a combined German, Austro-Hungarian, and Bulgarian offensive in 1915 resulted in Serbia's defeat and occupation.

The Eastern Front and the Balkans, though often overshadowed by the Western Front, were crucial theaters of war during World War I. The battles and campaigns in these regions influenced the overall strategy and outcomes of the conflict. They demonstrated the complexities of alliances, rivalries, and nationalist aspirations that contributed to the outbreak of the war and shaped its course.