You're right. It has to do with the Revolutionary War. Valley Forge, to be exact.
The American Crisis, Thomas Paine, December 23, 1776:
THESE are the times that try men's souls. The summer soldier and the sunshine patriot will, in this crisis, shrink from the service of their country; but he that stands by it now, deserves the love and thanks of man and woman.
The "summer soldier" that Paine wrote of was the person who didn't mind playing soldier when the times were easy, but couldn't hack it when times got tough.
"Winter soldier" was the opposite of "summer soldier." The Winter Soldier toughed it out during the hard times, but stayed true to the cause of Independence.
-PJ
Nice post!