World War I involved one group of countries fighting another.
Triple Entente . The understanding between the three powers , supplemented by agreements with Japan and Portugal , constituted a powerful counterweight to the Triple Alliance of Germany , Austria-Hungary and the Kingdom of Italy , though Italy did not side with Germany and Austria during World War I and joined the Entente Powers instead , in the 1915 Treaty of London . World War I . Being outnumbered on the eastern front , Russia urged its Triple Entente ally France to open up a second front in the west .