It is my contention that the Royal Navy and the French Army won World War I. The Royal Navy were past masters at blockading, it is how they dealt with all continental wars in the centuries past and only the American Revolutionary War slipped past their grasp using this strategy. The British sea blockade and their halting of the German ability to source materials through neutral nations placed direct pressure on German ability to expand their war economy.
The French Army was important as it bore the brunt of the fighting and the largest amount of the front. The French ability to absorb the German offensive in March of 1918 and stop it from reaching Paris or the coast is what saved the allied cause. After that the German back was broken and the pressures of lack of manpower, materials and food all came together to collapse Germany's ability to conduct war.
William H. McNeil writes in The Pursuit of Power that France was also the arsenal of democracy in World War I. (more)

