Because of this declaration of war on Serbia, Russia and its empire became involved, because of their alliance with Serbia. A chain reaction, Germany declared war on Russia, because of its alliance with Austria-Hungary. Britain became involved because of Germany’s invasion of Belgium, which at the time was neutral. Britain also had pacts with France for protection in the event of a war.
Because of Germany's geological position, if they were to start a war, than they would be fighting on two fronts: the Russians on the Eastern, and France and the U.K. on the Western (see map below). Throughout the war, the Triple Entente had approximately 8.6 million soldiers serving on the Western Front and Eastern Fronts. This is in stark contrast to the Triple Alliances’ 11.9 million strong armies spanning the two war fronts.
On the 6th of April, 1917, the United States declared war upon Germany, being, until that point, neutral. The declaration was on the grounds that Germany had violated its neutrality by attacking American international shipping convoys, resulting in American citizen deaths. Later on in December, the United States declared war on Austria-Hungary, entering the war as an ally to the Triple