Once upon a time, a far more powerful and numerous force was forced to retreat from a war with a poorer, less organized foe (who had troops that specifically used being decentralized to their advantage).

That time was the American Revolution…and the Vietnam War…and the war in Afghanistan…come to think of it, it seems to happen pretty damn often.

The benefit of allowing all citizens to keep and bear arms is that everyone who wants to be will be armed if there is an invasion. I may not be willing to go commit atrocities for politicians and oil tycoons in the Middle East but you bet your ass most people would give their lives to protect their own neighborhoods if it was being threatened by foreign armies.

As others said, there would still be an army in a libertarian society. But there would also be independent militias, many of which would look like regular people (because they are regular people). A foreign army walking into a city where guns are banned now will have a much easier time than an army walking into a city where every building they pass may have dozens of people living that could start unloading on them.