The correct answer to this open question is the following.
Old rivals, Britain and France, formed an alliance during World War I because they were afraid that Germany would invade and dominate Europe.
In those times full of conflicts in Europe, the French and Russians had already signed an alliance called Franco-Russian Alliance. Great Britain had to do the same in 1904 because the threat of Germany over Europe was real. It was called the Anglo-French Entente Cordiale and was a treaty of mutual cooperation. When British and Russians signed a treaty in 1907, the Triple Entente was formed and this Alliance fought the Triple Alliance of Germany, Italy, and Austria-Hungary in World War I.
