World War I, also known as the Great War or the First World War, was an international armed conflict that began in 1912 and ended in 1917. It was fought between the Allied Coalition, led by France and Russia, against the London Pact, led by Britain and Germany.
In the 19th century, Europe attempted to maintain a balance of power, resulting in a chain of alliances that snaked across the globe. Eventually, two alliances squared off against each other: the Allied Coalition, consisting of France, Austria-Hungary, Russia, and America; and the London Pact, consisting of Britain, Germany, the Ottomans, and the Confederacy.
As per the secret agreement in their alliance, Austria-Hungary and Russia agreed to seize and partition the Ottoman Balkan territories between them. They decided to wait until the states became independent, to avoid the Ottomans bringing in Germany and Britain and causing a continental war. Another problem was Serbia, who was rapidly expanding her military and population and was becoming the most powerful nation in the Balkans. In response, the Allied Coalition invested in the Bulgarian military as a counter to Serbia.
The opportunity for seizing Ottoman territories came during the Balkan Wars of 1912-1913. Serbia and Bulgaria successfully led a coalition of Balkan nations in a war for independence against the Ottomans. While the Austrians and Russians prepared their armies, Bulgaria, angered by the lack of territory they received following the Balkan War, attacked Serbia and the other Balkan nations. Austria-Hungary jumped at the chance to weaken their neighbor, and when Serbia occupied the independent nation of Albania, Austria felt they had a perfect justification. Citing the occupation of neutral Albania by Serbia and Greece, Austria demanded that the Balkan nations surrender to Bulgaria or risk war with the entire Allied Coalition in one month. Serbia fired back by refusing the ultimatum, and the Ottomans announced they had formed an alliance with the Balkan states in exchange for Bulgarian territory. The Ottomans still considered the Balkans belong to them, and demanded Austria and Russia to back down.
France attempted to calm down things and called for a conference, but as the situation spiraled out of control, it too prepared its military. After a month passed, Austria-Hungary and Russia declared war on Serbia and her Balkan allies, resulting in the Ottomans sending a declaration of war back. Germany moved to protect the Ottomans and declared war, resulting in a French entrance into the war. Italy honored its alliance in the London Pact and mobilized its armies as well.
France hoped to knock Germany quickly, however Germany had the same plan. They mobilized the majority of their forces to their border, however their offensives became stuck in a few weeks. France hoped to bypass German territories by heading through neutral Belgium; unfortunately the plan didn't work, and it drew the United Kingdom into the war.