The last time the french won was during the Napoleonic War, after that the Germans kicked their asses. And WW1 doesn't really count to me because it was a trench-line war with no gain on both sides (taking only the germans vs french/british). The time of the french conquerring anything ended with Napoleons 2nd Exile.
The First Battle of the Marne was not trench warfare. In fact the opening few battles didnt see the stereotypical trenches that is usually shown in pictures of WW1. You do a great disservice to history when you say that the French did not fight heroically in WW1. If it wasnt for French soldiers heroically holding the line at places like the Marne and especially Verdun (the longest battle in history and probably one of the most gruesome), the Germans probably would have won WW1.
And they where able to hold the line after Winter '15 because of what? British reinforcements and supplies. And even the British couldn't change the tide, but only pin down the Germans. It needed the USA and their vast military machinery to break the German front. Though I give a great deal of the Allied Victory not to military success but simply to the embargo of German trading routes. There were no 'heroic' acts in that war. Just advance and hold. It was a pyrrhic lose and gain game for both sides.