In vietnam 175 marines held a small airfield against 1500 trained North Vietnamese. They had fewer rounds of ammunition than there were enemies, yet with the help of minimal air support and artillery they killed every single one of them.
That is a perfect example of a trained fighting force with massively superior numbers being defeated by a professional army.
No, thats a perfect example of the fact that Marines can do the impossible.
On a side note, if a 'country' were to become Anarchist, and the rest of the wrold was not, it would alsmot definitly be invaded, and some form of standing defense force would be needed. However, that military would have to be paid in some way. If nothing else, they would need a supply of ammunition. In order for them to get that ammunition there would have to be some way for the military to get money. So, it would have to either steal money, or collect taxes. Which becomes a small problem.
Now, even if the whole world were to become Anarchist, There is still the unavoidable fact that eventually someone is going to start a gang to go around killing/stealing from people (very Road Warrior like). Or, one town will just organise to take over it's neighbor. Again, a standing defense force would be needed.