The lack of effective methods to measure the combat effectiveness of an army deters military mathematical models and military simulation software from realizing their full potential. In this paper, we use Galilean idealization and domain theory to suggest solutions and propose an appropriate mathematical method to reach universally applicable results. We introduce the International System of Units (SI) to measure combat effectiveness without relying on subjective assumptions regarding parameters. Our method requires only spatial and temporal data to calculate combat effectiveness by evaluating the extent of territorial changes (outcome) within a given time period (time). Moreover, we measure influencing factors of combat effectiveness, based on the ratio of combat effectiveness. We discuss the practical value and applicability of this method in the development of the analysis-simulation software of combat effectiveness.