Early in Russia’s invasion of Ukraine, it wasn’t just Moscow that believed its offensive could succeed quickly. In February, even U.S. officials warned Kyiv could fall in days. Russians had numbers on their side, or more precisely a number: the 3:1 rule, the ratio by which attackers must outnumber defenders in order to prevail. It is one of several “force ratios” popular in military strategy. Russia, it seemed, could amass that advantage.