Well there are "go pills" :
dextroamphetamines - more commonly known as 'Speed', but referred to as 'Go Pills' within the USAF - to its pilots to ensure they remained alert long after tiredness and fatigue should have started to impinge on mental performance.
Although the benefits of such drugs have been rendered ideal, the side effects, including confusion, delusions, auditory hallucinations, aggression and, in extreme cases, psychotic behaviour, represent a disposition that is hardly ideal for a person in charge of a multimillion-dollar war machine. This is best evidenced by an incident in April 2002, in which US pilots became involved in a friendly fire incident over Tarnak Farms, Afghanistan, while under the influence of Dexedrine. Four Canadian soldiers died in the accident.
DMAA:
The US Army sought to follow the USAF's lead regarding the use of performance enhancing drugs by investigating Dimethylamine (DMAA), a performance enhancing supplement registered on the World Anti-Doping Agency's banned substance list. Investigations were under way until the supplement was pulled from the shelves, having been linked with the deaths of two soldiers in December 2011 while undergoing a routine training drill.
Supplements containing DMAA grew in popularity due to their premise of increasing stamina and performance, particularly in preparation for gruelling physical challenges. The drug captured headlines in the UK after it was linked to the death of 30-year old Claire Squires, who died of cardiac failure having run 25 miles of the 26.2 mile London Marathon course.
DARPA study:
The comments regarding the US DoD's interest in stimulants harks back to DARPA's Peak Soldier Performance Programme, which in 2004 sought a biochemical approach that would allow a soldier to operate in theatre for up to five days without requiring sustenance. In pursuit of this, no stone or genome was left unturned.
DARPA's approach included investigations into endurance-building nutrients that could be taken in the lead-up to deployment, while also lowering a soldier's core temperature and boosting his or her mitochondria. Mitochondria are effectively a cell's power supply, fuelling it by converting sugars into chemical energy. Oxford University biochemists sought ways in which mitochondria could be altered genetically to have them feed off fat-based ketones, leading to much smaller, ketone-based rations being capable of not only keeping soldiers on their feet, but having them operate at their peak for days at a time. Initial laboratory tests demonstrated that rats given the treatment were able to run for extended periods of time.
Other approaches include altering a soldier's metabolism to reduce the need to feed, lowering body temperatures during strenuous activities, so that less energy was exerted, and even caffeinated gum, which would provide a quick energy boost in times of dire need, was experimented with. The idea, as one DARPA official informed Wired, was not to create Supermen, but to make it so that "these kids could perform at their peak, stay at their peak, and come home to their families."
Anything more than that is probably classified and unavailable to the public, but the study above would seem to suggest that in addition to GMO's we'll have GMS(oldiers)'s next and what could go wrong?