World War I, also known as the First World War, the Great War, and the War to End all Wars, was a global military conflict, which took place primarily in Europe from 1914 to 1918. Over 40 million casualties resulted, including approximately 20 million military and civilian deaths. The conflict had a decisive impact on the history of the 20th century.

On August 1, 1914 Germany declared war on Russia, after Russia refused to halt mobilization of its forces in support of Serbia. France, bound by treaty to Russia, found itself at war with Germany. Germany then declared war on France on August 3, 1914. Determined to reach Paris by the shortest possible route, Germany invaded Belgium and Luxembourg. France then launched its own assault against Germany into the area south of Metz (a city in the northeast of France) to meet the advancing German army.

France, the Russian Empire, the British Empire, Italy and the United States were to be known as the Allied forces. While France, Russia and Britain entered the war in 1914 as a result of the Triple Entente (their alliance of 1907), the U.S. entered the war as an ‘associated power,’ rather than as a formal ally of France and Britain, and maintained that stance for the duration of the war. Many other countries later joined the Allied forces.

In 1914 the Russian government considered Germany to be the main threat to its territory which was reinforced by Germany’s decision to form the Triple Alliance. Under the terms of this military alliance, Germany, Austria-Hungary and Italy agreed to support each other if attacked by either France or Russia. In August of that year, the Russian army entered Germany to support the French army.

U.S. President Wilson declared a policy of absolute neutrality, an official stance that would last until 1917. Germany’s policy of unrestricted submarine warfare and further U-boat attacks on American merchant ships, finally forced the U.S. to enter the war on April 6, 1917.

Britain, allied to France by treaty, was under moral obligation to defend France and so declared war on Germany on August 4, 1914. She was also obligated to defend neutral Belgium by the terms of a 75-year-old treaty. With Britain’s entry into the war, her colonies and dominions, including Australia, Canada, India, New Zealand and the Union of South Africa, variously offered military and financial assistance.

The fighting that took place along the Western Front occurred along a system of trenches, breastworks, and fortifications separated by an area known as “no man’s land.” These fortifications stretched 475 miles (more than 600 kilometers) and defined the war for many.

Italy, although allied to both Germany and Austria-Hungary under the Triple Alliance (a military alliance that lasted from 1882 until the start of World War I in 1914), was able to avoid entering the war by citing a clause enabling it to evade its obligations to both. In short, Italy was committed to defending Germany and Austria-Hungary only in the event of a ‘defensive’ war. Arguing that their actions were ‘offensive’ she instead declared a policy of neutrality. The following year, in May 1915, Italy entered into a secret agreement with Great Britain, joining the Allied forces in the conflict against her two former allies.