1. Germany as the aggressor moved first. The Schlieffen plan almost worked as they came around the back of Paris. Also their industrial and military strength.
2. USA entered the war late. They were fresh and ahd not been physcially under attack for as long as the other allies had
3. Allies in general had the advantage of presenting multi-fronts
4. Englands naval supremacy and their isolation by the English channel, their empire/colonies
5. Russia - it's massive over 6 years ago