My Dad (32nd Infantry Division, New Guinea) took up smoking when he got to the combat zone and gave it up when he came home. He said that the cigs were about the only comfort and the only stress reliever they had, and that the troops would have managed a day without food better than a day without tobacco.
Our three sons, all Marines, served in Iraq. All smoked while they were active duty and deployed. All three quit shortly after being home for good.