Well, for all of it's history, the US has the great oceans (pacific and atlantic), that would require many months travel for enemy invasions. There was a joke that would say "how many british soldiers does it take to fix a light bulb"? 3. One in land and the other in the ocean. British communication was severed by it, as well as confusing strategy positions since they had to get updates from England. The 13 colonies also were in a large region, requiring british troops to occupy in many lands. Sickness' also developed in the south.