This article surveys the employment of geology in war with special reference to its tactical application. Approaching the subject theoretically, from a military rather than an academic viewpoint, the first problem examined is whether our present undistinguished use of geology in war results from some inapplicability inherent in the science itself, or whether a more rational analysis will not disclose wider fields of application by recognition of hitherto unsuspected correlations with the principles of war. After preliminary definitions of war, the principles of war, geology and military geology, the development of military geology is outlined to show what aspects have been most useful and the phase of warfare to which they are best adapted. This reveals that there has been but little actual tactical application of geology in mobile warfare or a war of maneuver, and that its employment has not been worked out satisfactorily in the United States Army, although it has been developed to some extent by the Germans. Hence in our current engagements geology seems not to be utilized fully because of the erroneous belief that it cannot be applied with sufficient rapidity to keep pace with the course of modem combat. In order to bring the soldier and the geologist into mutual understanding, military and scientific thought are compared and shown to follow substantially the same outline. Then, terrain is considered from the viewpoint of army doctrine and geology, and emerges as the common denominator of geology and war. With terrain as the connecting link geology can be correlated with the principles of war. When military leaders are convinced that these correlations are valid and contribute to the strength of the principles of war, more widespread applications of geology to war will follow, and geology will rightfully take its place among the “front-line” sciences. Other conclusions are that, as the principles of war are immutable, and do not vary with change of weapons or tempo of combat, it follows that geology can keep pace with modem war in its most fluid tactical phases. Geology makes its chief contribution to war through the principles of Mass (Superiority) and Economy of Force, the fundamental law of war; but the principles of Movement (Mobility), Surprise, Security, and Simplicity are also strengthened through its application. The total effect of the application of geology to these principles is superior information—upon which the possibility of success is principally dependent. Some suggestions are made regarding the geologic training and military qualifications of the modem military geologist. Finally, when all other things are equal, victory will come to him who makes best use of the ground.