Concerned that Germany was a threat to its territory, Russia joined World War I to prevent the geographically smaller nation from gaining a foothold on its borders. Seven years prior to the war, Russia joined France and Britain as allies in the face of the formation of the Triple Alliance, which comprised Germany, Italy and the Austro-Hungarian Empire.
When World War I began, General Alexander Samsonov, commander of the Russian Second Army, invaded the German province of Prussia by crossing its southwest border. His goal was to reach the middle of the province, where Russian General Paul von Rennenkampf was heading from the northeast.
Although the Russian Army was the world's largest at the time, the country's substandard railroads and roads made effective invasion challenging. Before it could get far, the Second Army found itself surrounded by German soldiers. Only 10,000 of 150,000 Russian soldiers managed to escape; the rest were captured and held as prisoners or killed. Samsonov, devastated by the defeat, committed suicide.
One year into the war, Russia, which lacked enough weapons for its large military, had lost two million of its 6.5 million soldiers. It also lost Belorussia, Kurland and Lithuania.
By March of 1918, as German troops advanced on Petrograd, Vladimir Lenin, the nation's leader, ordered Leon Trotsky, who served as leader of the nation's army, to sign the Brest-Litovsk Treaty. Russia thereby surrendered Poland, the Ukraine, the Caucasus, the Baltic provinces and Finland.