Assuming you're taking about the effects of fasting, there are exceptions to fasting. You do not have to fast if:

(Probably forgetting a few things here)

• You're traveling
• You're pregnant
• You don't have good health
• Aren't in a stable condition to do so (uncertain of the next time you can afford to eat)

Military campaigns count as traveling and as an unstable condition so a soldier would be exempt from fasting, but can fast if he still desires to do so.