World War I: The First Russian Army, led by Paul von Rennenkampf, enters East Prussia.

World War I or the First World War, also known as the Great War, was a global conflict between two coalitions: the Allies and the Central Powers. Main areas of conflict included Europe and the Middle East, as well as parts of Africa and the Asia-Pacific. There were important developments in weaponry including tanks, aircraft, artillery, machine guns, and chemical weapons. One of the deadliest conflicts in history, it resulted in an estimated 30 million military casualties, plus another 8 million civilian deaths from war-related causes and genocide. The movement of large numbers of people was a major factor in the deadly Spanish flu pandemic.

The 1st Army was an army-level command of the Russian Imperial Army created during World War I. The First Army, commanded by General Paul von Rennenkampf, invaded East Prussia at the outbreak of war in 1914 along with the Second Army commanded by General Alexander Samsonov. After declaring war on the German Empire, the Russian Empire had been able to mobilize very quickly. All Russian forces were put under the command of Grand Duke Nikolai and his Quartermaster General Yuri Danilov.

Paul Georg Edler von Rennenkampf was a Baltic German nobleman, statesman and general of the Imperial Russian Army who commanded the 1st Army in the invasion of East Prussia during the initial stage of the Eastern front of World War I. He also served as the last commander of the Vilna Military District.

East Prussia was a province of the Kingdom of Prussia from 1772 to 1829 and again from 1878 ; following World War I it formed part of the Weimar Republic's Free State of Prussia, until 1945. Its capital city was Königsberg. East Prussia was the main part of the region of Prussia along the southeastern Baltic Coast.