several things to consider with this problem, the navy is important if you are invading a country with a strong navy or no friendly airfields nearby, because from a purely tactical standpoint, the air force can do the most damage to high priority targets, but is useless if it cant get there. On the other hand i don't believe you can really win a war without getting ground troops in their, because with an exclusive air campaign, particularly in modern day conflicts us Americans are getting into, the "death from above" will mostly fall on civilians.