The Triple Entente consisted of Great Britain, France and Russia. It was formed through a variety of different diplomatic agreements and treaties between the three countries in the late 19th and early 20th centuries, and helped smooth over previous rivalries the three countries had with one another. The Triple Entente stood as a counterbalance to the Triple Alliance of Germany, Austria-Hungary and Italy.
The Triple Entente officially began when France and Russia, fearing isolation on the European stage, formed an alliance in 1894. Great Britain joined the "Entente Cordiale" in 1907, officially uniting the three old enemies. The alliance agreement did not obligate any of the parties to go to war on behalf of the others, but merely specified a "moral obligation" to support one another.
The alliance entered World War I in 1914. Russia was the first to declare hostilities when Austria-Hungary invaded Serbia. Russia had been building up its military forces for some time, but officially mobilized on July 30, 1914, prompting Germany to declare war against the country. France's alliance with Russia brought it into the fight immediately. Great Britain, on the other hand, did not enter the fight until Aug. 4, 1914, in response to Germany's invasion of Belgium.