The U.S. Department of Defense has had to assure combat troops in Iraq and Afghanistan that they would not be deprived of their tobacco products. That reassurance was necessary because of a recent announcement that a Department of Defense study had recommended that the military become tobacco-free. That process would take, it is believed, at least twenty years. That's because it would be done gradually. And even then, cigarettes might still be retained as a battlefield approved drug. There are many such drugs to help troops deal with combat stress and fatigue.
This fatigue problem has existed for a long time, and has become particularly acute in the last century or so, as battles became endurance contests, with forces engaged for days on end. In the last few decades, as long range bombers, and refueling in the air, became common, pilots have had to face the same problem during very long (30 hours or more) missions. For over a century, the solution has been amphetamines ("speed"). However, this drug can impair judgment, making the user more aggressive, for example. In the last decade, kinder and gentler medications have become available. While the new drugs do a pretty good job, dextroamphetamine is still a bit better. So amphetamines remained competitive.
Wakefulness can be a potent weapon, especially for commandos, or troops engaged in prolonged combat (like the Battle of Fallujah in 2004). Without these wakefulness drugs, you would have to either pull troops out of action so they could rest, or leave them in and risk having them make fatal mistakes. Either way, you have a problem, because there are never enough troops to get the job done. But with the wakefulness medications, you can solve the problem, for a few days, anyway. Prolonged use of these drugs is not healthy. But neither is being drowsy during combat.
It's also become common to prescribe other types of mood altering drugs, like antidepressants and tranquilizers to combat troops, to help them deal with stress. But for short term relief, on the battlefield, nothing beats a cigarette. Just like coffee is a good short term alternative to amphetamines, tobacco (chewed or smoked) gives momentary relief during lulls in combat. A third of military personnel smoke, compared to a fifth of their civilian peers. In some combat units, up to half the troops are smokers (if only for as long as they are in the combat zone.)
The troops know that the drugs they take have bad side effects, but combat is inherently dangerous, and staying awake and stress free is the easiest way to avoid all the other dangers.