A post-World Wars Western (read European or nation-states settled by Europeans) reluctance to accept casualties in the prosecution of a war (note: this does not apply to non-Western countries which often suffer very high casualties)
It is obvious to any student of history that post-World War II Western military success is defined in terms of Western casualties suffered and not military/strategic objectives achieved. Think about it this way. We can all probably agree that the perception is that the people in the West will not tolerate high casualties in military operations. That is conventional wisdom at least since the 1970’s. I am not convinced it is true. I think if the case were made, then people would be willing to accept a casualty if THEY THOUGHT THE SACRIFICE WAS GOING TO ACHIEVE SOMETHING!
Remember the angst-ridden editorials at the height of the Iraq War in the spring and summer of 2007 when casualties were running above 100 combat deaths some months and made 2007 the deadliest year for US forces. Here are some examples from Reuters, The Atlantic, Counterpunch, and the Boston Globe just as examples. USAToday actually has a balanced piece about casualties on their website.
I believe that the main driver of casualty aversion is that military operations seem to fizzle because of lackluster prosecution. If the case was properly made by the national leadership as to why Western forces were engaged in conflict it is probable that the public would more readily accept casualties to achieve a definable goal. Casualty aversion is a direct result of the failed wars fought by the West since the end of World War II. It is entirely reasonable for the public to ask why they should accept high casualties when it appears that soldiers are fighting and dying in lost causes or wars that end unsatisfactorily from a public perspective such as Korea, Vietnam, Iraq, Algeria, Belgian Congo, Somalia, etc.
Why is it that leaders and policymakers seem unable to elucidate and make the case for specific quantifiable war goals? The public is correct to demand that the lives of soldiers not be squandered to achieve nebulous goals or even to be wasted in a cause not decisively brought to a conclusion. The military will fight, but it is the role of the public to demand that lives only be sacrificed in what the people believe is a worthy cause.
The failure is in the formulation, execution, and explanation of war goals. There is nothing concrete about “nation-building”, R2P, exporting democracy, humanitarian assistance, amorphous “terror”, or any of the other numerous reasons soldiers have been put in harm’s way in the past 70 years. Give the Western public a concrete enemy and a logical reason that enemy must be defeated and they will wholeheartedly support the effort to defeat said enemy. That is apparently the one thing Western politicians are incapable of doing.
 “ICasualties | OIF | Iraq | Fatalities By Month.” ICasualties | OIF | Iraq | Fatalities By Month. Accessed November 23, 2015. http://icasualties.org/iraq/ByMonth.aspx.