In this war, the French were on the side of the Allied Powers. They were one of the two most important countries on this side of the war.
World War I was fought between the Allied Powers and the Central Powers. The most important of the Allied Powers were, at first, France, Britain, and Russia. Later in the war, the US entered the war and Russia left it. The most important Central Powers were German, Austria-Hungary, and the Ottoman Empire.
The French were allied with the British and the Russians because they all feared the increase in German power that was occurring at the time. They made an alliance against Germany. When Russia was pulled into the war, France and England honored their alliance and joined the war as well. France and England were the two main countries fighting Germany on the Western Front from 1914 until the US entered the war in 1917.