You sure about that? http://militaryatheists.org/
Still, the phrase "there are no atheists in war" is an argument against religion, not against atheism. The mental capacity for thinking and decision making of a desperate soldier fighting for his life is hardly going to be more rational or logical than that of a calm person not at risk of death. To say that you become religious when you are at your most irrational is not an argument in your favor.
You really don't have to guess at it... pretty much anything you can think of, someone has already done the research for you... google it
According to this study, 28% of WWII soldiers never prayed, even during combat... that's a lot of atheists if you ask me