At the beginning of World War I, East Prussia became a theatre of war when the Russian Empire invaded the country. The Russian Army encountered at first little resistance because the bulk of the German Army had been directed towards the Western Front according to the Schlieffen Plan. Despite early success and the capture of the towns of Rastenburg and Gumbinnen, in the Battle of Tannenberg in 1914 and the Second Battle of the Masurian Lakes in 1915, the Russians were decisively defeated and forced to retreat. The Russians were followed by the German Army advancing into Russian territory.

Which country invaded east Prussia in World War I?