The ubiquity and evolving nature of cyber attacks is of growing concern to industry and society. In response, the automation of security processes and functions is the focus of many current research efforts. In this talk we will present a framework for automated network intrusion response, in which we model the interaction between an attacker and a defender as a partially observed Markov game. Within this framework, reinforcement learning enables the controlled evolution of attack and defense strategies towards a Nash equilibrium through the process of self-play. To realize and experiment with the self-play process on a practical IT infrastructure, we have developed a software platform for creating digital twins, which provide two key functions for our framework: (i) a safe and realistic test environment; and (ii) a tool for evaluation that enables closed-loop learning of security strategies.