Opportunity:   Sadly, wars cause many people to suffer serious wounds in a short period of time.   Further, throughout history, war is known for breeding disease in epidemic proportions.  At least five times as many people died during the American Civil War from disease than in combat.  The unsanitary conditions of soldiers grouped in masses, often with rotted food, polluted drinking water, and living in their own filth, gives rise to much illness.  Also, soldiers traveling great distances may be exposed to diseases for which they have never acquired any immunity.These traveling soldiers themselves could even bring new diseases to vulnerable local populations.  There is no doubt that wartime doctors are confronted with huge numbers of sick and broken bodies, a sad laboratory, but well stocked indeed with subjects.   Further, in the absence of adequate time and normal supplies, wartime doctors often are forced to "make do" with whatever they can think of, or even try new, unfamiliar methods used by the local population. Clearly, war provides ample opportunity to explore new medical techniques.  Look for examples of this throughout this website, such as with Head Injuries in Ancient Egypt and improvements in medicine as a result of the gruesome American Civil War.
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Motivation:   Wartime can be very motivating!  Individuals often do everything they can to help a war effort, perhaps out of patriotism, a desire for glory, or maybe from just the simple drive to survive!  This extends easily to a motivation to provide the best possible medical care to one's soldiers.  This is both out of compassion, and to keep them fighting better and longer.  Governments and military organizations, if well-organized (see below) and somewhat enlightened, can be very motivated to ensure the health and survival of their fighters, and so may sponsor rapid research programs.   Consider the extremely rapid discovery of ways to manufacture penicillin during World War II and fight yellow fever in the Spanish-American War .
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Organization:   Apparently, not all wars lead to significant medical innovations.  If no medic or doctor is assigned to troops, and soldiers treat their own wounds, how easily will any successful methods be passed on?  Even if doctors are in attendance, if there is no system of communication between them, how will any discoveries be spread around?  No better example than Ancient Rome shows that when a government and an army become organized, and when that organization dedicates time and resources to the best medical care that it can provide, wars do lead to innovation.

Dealing with diseases:   Throughout history, more soldiers are killed by disease during war than by combat, typically by massive epidemics, like typhoid fever and yellow fever, that strike the densely packed warriors.  Estimates range as high as ten to one for some wars (Crimean and Mexican Wars), a ratio of five to one has been typical.  Only since World War I has medical science learned enough to reduce disease-based war deaths to fewer than the number of combat deaths.  There are at least three different ways that wartime doctors have sought to fight disease:

Dealing with injuries:   Although disease has historically killed more soldiers during war, wounds from battle are perhaps more dramatic and immediately disabling.  War may be associated, mentally, more with severe injury, and historically wartime doctors have been recruited to tend to wounded warriors.  It is not surprising that many of the battlefield medical innovations have been about how to rapidly and effectively deal with injured bodies.