How was Giuseppe Garibaldi able to conquer The Kingdom of Two Sicilies with only a few volunteers?

by edenapple
IonicSquid

And if it's not too broad to warrant its own thread, are there any other examples of successful invasions (rather than internal coups) staged against a much larger force with only the metaphorical (I'm sorry) twenty good men? It seems like a monumental feat to sweep in with a relatively tiny army and conquer a region with a much greater amount of manpower available to it.