A historical example to support this statement?

Question

"Fortune is the arbiter of half the things we do, leaving the other half or so to be controlled by ourselves"

Answer 1

Luck plays a very important role in just about everything we do.  In fact, I'd consider myself lucky if I can "truly and completely control" just half of the things we do.

Historical example:  The American Army during the Revolutionary War was extremely lucky.  Luck played a key role in saving the American army - when fog prevented the British from seeing that the Americans were leaving in the middle of the night, or when a storm temporarily halted the fighting at a time when the Americans desperately needed a day's rest.

Answer 2

Napoleon's army was half-beaten by the Russians and half-beaten by the weather.