Final answer:
Caffeine was frequently used in the military to keep soldiers awake and fight fatigue due to its properties as an adenosine antagonist, which decreases sleepiness and increases alertness.
Step-by-step explanation:
The substance that was used in the military frequently to keep soldiers awake and fight fatigue is caffeine. Caffeine is a well-known stimulant found in coffee, tea, and even some pain relievers. It functions as an adenosine antagonist by inhibiting the adenosine receptors, which are part of the neurotransmitter system that promotes sleep. Consequently, caffeine consumption leads to decreased sleepiness and increased wakefulness, helping soldiers remain alert during demanding situations.
Beyond its use in the military, caffeine's ability to increase alertness has made it a common ingredient in various products aimed at combating drowsiness. Caffeine has also been utilized in conjunction with other medications, such as antihistamines, to counteract the drowsiness they may induce.