The difference between aid, treaty, and alliance is that an aid is a help of a practical nature and a treaty is a agreement between countries and a formally concluded. An alliance is a union or association made up for mutal benefit. Its made up especially between countries or organizations. A treaty is not as important as a aid that goes and fight for what they want and an alliance is they help each other because their like co-workers that work together.
Part 2:
The U.S isn't part of that diagram because their war didn't start until April 6, 1917. When the U.S. enter and joined its allies Britain, France, and Russia to fight in World War I. All under the command of Major General John J. Pershing.The U.S entered the war on April 6, 1917