There are several reasons why this should be true and why it applies especially to the United State today.
To start with, the idea of an invasion of the United States is absurd. Our North American redoubt is virtually invulnerable to attack and there is no enemy which could even think of attacking, even if our armed forces were close to non-existent. As I said, a few warheads to pay homage to the God of MAD are quite sufficient.
It is possible that in, say 2020 or 2030, China could develop a real threat. If so, then deal with it when it happens. Why waste money on hardware today? It'll be obsolete before it could possibly become useful. It also retards the growth of our economy.
War has also become close to obsolete. National boundaries are pretty much inviolate. These days empires break up instead of being assembled. The last country to be annexed was Tibet. 42 years ago. Even in this case, China felt obliged to make to cover up the invasion by claiming that it is actually belonged to China, a claim that does, in fact, have some (weak) historical merit.
Many countries do not have armed forces. One good example is Costa Rica. Not by coincidence, the place is far more peaceful than any of its neighbors.
The observation has been made that democracies do not go to was with each other. This is probably because they need to demonize the enemy in order to wage war, as Hoppe observes. In fact, democracy makes a country virtually immune to invasion as well. The last invasion of an advanced country goes back to the Second World War.
Conclusion: armed forced are not really necessary, especially not here. They do, however, constitute a serious provocation to other people and nations. And they do represent a serious threat to our liberty. War is the health of the state.