Evidence of the Law of Motion
Changes in National Wealth Due to War
Some initial evidence for the law of motion can be found in historical time series of national wealth, in which major destructive events tend to leave their mark. Consider the following chart of German national wealth from 1886 to 1968. It features two major dips coinciding with each of the World Wars, 1914-1918 and 1939-1945, when German strength was severely weakened due to the combat. Germany's wealth plummeted rapidly around 1943-1944, after the Soviet Union turned the tide at Stalingrad and the other Allies opened up a second front in the west.
A similar patterns holds for France, which also suffered devastation during those wars:
But obviously not every decrease in national wealth can be attributed to war. For example, in the chart below of U.S. national wealth from 1870 to 2017, the major dip occurring from 2008 to 2013 was due to financial collapse rather than international conflict. There are dips coinciding with the World Wars, but these are minor, reflecting the fact that the war was for the most part not fought on U.S. soil.
Though more impressionistic than dispositive, these time series suggest the almost tautological observation that the application of destructive power tends to decrease the recipient's power, as measured by national wealth.