An excellent question....this is the first question we discussed in my new class:
In particular, Clausewitz understood that war often went in unanticipated directions, be it ‘friction’ or ‘chaos’ or unrestrained emotion leading to irrational thinking because people wage war, and people react to violence in unpredictable ways. Policy makers over the last 50 years have too often initiated warfare, or even limited military action without thinking through all of the downstream effects that military action can create.
Because warfare in the days of Thucydides, Clausewitz and Sun Tzu was literally a matter of life or death for a country or dynasty, rulers had a much better appreciation of the risks of going to war.A ruler had to know what they wanted to accomplish by force of arms, and most importantly, how much blood and treasure they were will to expend to achieve those goals.
Current leaders would do well to understand Clausewitz’ trinity and realize that if this balance is not maintained, the use of military force to achieve diplomatic purposes will not be as successful.