France was one of the Triple Entente powers during World War I along with Russia and England. In the long run, actions in World War I were bound by treaties. France had a treaty with Russia, who in turn had a treaty with Serbia. Joseph Joffre was the French Commander in Chief for much of World War I.
Yes much of World War I was fought in France with some of the bloodiest battles of the war occurring there. In addition, all of the Western Front was in France. The Allies (Mostly Britain and France) suffered 8.6 million casualties, the Central Powers (mostly Germany) had around 4.7 million casualties.