A reluctance to prosecute war to the extent necessary to achieve victory even when a realistic definition of victory was elucidated.
This one should be a no-brainer as recent American experience has shown that stupidity very much exists at the top of American strategic thinking at least, which is compounded by clueless media talking heads who I am more and more convinced actively wish to see Western society fail. Clausewitz says that war naturally tends to extremes but in reality never gets there.
This point goes back to deciding what determines victory. Clausewitz is undoubtedly correct in his assertion that ultimately victory is in fact using force to compel the enemy to do your will. This is regardless of what that will is whether it be forcing a nation to cede territory, stop a particular action, or making terrorists stop launching terrorist attacks. The extremes part comes in because the level of violence applied must be continually escalated in order to achieve dominance. Further it must be applied in such a way that the minimum force necessary must be applied at the decisive point to achieve dominance.
Exactly what the decisive point is will be different in every instance. The decisive point is whatever aspect of the enemy or his defense that can be attacked that will cause them to submit to your will. In traditional combat this may be the fall of a certain position, outflanking a formation, killing a commander, cutting supply lines, or whatever other action will cause enemy resistance to collapse.
Now pause and ask yourself, what was the last major war in which a Western power achieved a decisive victory? My answer is the 1973 Yom Kippur War between Israel and a coalition of Arab nations. It was decisive because the Israelis beat the Arabs so badly that their coalition collapsed and no Arab nation-state has since challenged Israel in conventional warfare, instead they have sought to destabilize Israel through proxy groups such as Hezbollah, Islamic Jihad, etc. The last decisive European victory was in 1945 with the defeat of Germany and Japan although I am undecided if the British victory in the Falklands War was decisive or not.
The reason for the lack of decisive war termination is directly related to the topic of the next post, the reluctance of Western nations to accept casualties. It is also ridiculously easy for Western nations to simply declare victory, or mission accomplishment which is the same thing, and take their marbles and go home as the US did in Vietnam in 1973 and Iraq in 2011. The US can get away with this because they have not fought an adversary that was an existential threat to the United States since 1815, if we really want to be honest about it.