The clip below of the Battle of Wits scene from The Princess Bride (IMDB), the character Vizzini (Wallace Shawn) says:

You’ve fell victim to one of the classic blunders! The most famous is never get involved in a land war in Asia, but only slightly less well known is this; never go in against a Sicilian, when death is on the line! Aha ha ha ha… (and then keels over, dead.)

What does the line "never get involved in a land war in Asia" refer to? I don't understand the reference. Does it connect to the plot in some way I'm missing, or is it completely unrelated?