It's true that the U.S. and Tibet are on opposite sides of that spectrum, but there are a number of countries whose military spending as a percentage of GDP is between 1/4 and 1/2 as high as it is in the US, but who nonetheless defend themselves credibly. Examples: France, Turkey, Italy, Australia, Canada.
I can buy that some proportion of military spending is therefore for protection, but the actual amount of military spending seems considerably higher than the amount needed to keep the U.S. from being colonized.