This paper describes a basic passive-versus-aggressive defense model and analyzes it in terms of defense strategies against an intelligent enemy. In response to varying combinations of passive and aggressive defense, we assume that the enemy can up or down regulate the growth activity. This leads to a differential game formulation of battle scenarios. First, we examine military counterterrorist activities in a civilian population. We then consider immune defense against hepatitis B in the liver. The underlying principles in these two examples are similar, although the model features are slightly different. Simulation results are provided to demonstrate the benefits and limitations of the proposed model in terms of understanding biological systems and developing strategic army defense plans.