1914 – World War I: Germany declares war against France, while Romania declares its neutrality. World War I, or the First World War (28 July 1914 – 11 November 1918), also known as the Great War, was a global conflict between two coalitions: the Allies (or Entente) and the Central Powers. Major theaters of war included Europe and the Middle East, and well as parts of Africa and the Asia-Pacific. One of the deadliest conflicts in history, World War I resulted in an estimated 15 to 22 million deaths, including in mass war crimes against civilians and genocides. The conflict saw important developments in weaponry, including the first large-scale use of machine guns, artillery, aircraft, chemical weapons, and tanks, and was a major factor in the deadly Spanish flu pandemic. The causes of World War I included the rise of the German Empire, which disturbed the long-standing balance of power in Europe, and the exacerbation of imperial rivalries, as reflected in new alliances and an arms race between the great powers. Tensions in the Balkans reached a breaking point on 28 June 1914 when Gavrilo Princip, a Bosnian Serb, assassinated Franz Ferdinand, the heir to the Austro-Hungarian throne. Austria-Hungary blamed Serbia and declared war on 28 July, upon which the Russian Empire mobilized in Serbia's defense. In early August, Germany declared war on Russia and France; the United Kingdom entered the war after Germany invaded Belgium. Germany's strategy in 1914 was to quickly defeat France, but its advance was halted in September, and the Western Front solidified into a near-continuous line of fortified trenches from the English Channel to Switzerland. The Eastern Front was more dynamic, but neither side gained a decisive advantage, despite costly offensives. The Ottoman Empire joined the Central Powers in November, which was followed by a failed Allied campaign at Gallipoli. Japan and Italy also joined the Allies. Major battles, including those at Verdun, the Somme, and Passchendaele, failed to break the stalemate on the Western Front. In April 1917, the United States joined the Allies after Germany resumed unrestricted submarine warfare against Atlantic shipping. Later that year, the Bolsheviks seized power in Russia in the October Revolution; Soviet Russia signed an armistice with the Central Powers in December, followed by a separate peace in March 1918. That month, Germany launched a spring offensive in the west, which despite initial successes left the German Army exhausted and demoralised. The Allied Hundred Days Offensive, beginning in August 1918, caused a collapse of the German front line. By early November, the Allies had signed armistices with the Ottomans and with Austria-Hungary, leaving Germany isolated. Facing a revolution at home, Kaiser Wilhelm II abdicated on 9 November, and the war ended with the Armistice of 11 November 1918.
