A barrier to developing novel AI for complex reasoning is the lack of appropriate wargaming platforms for training and evaluating AIs in a multiplayer setting combining collaborative and adversarial reasoning under uncertainty with game theory and deception. An appropriate platform has several key requirements including flexible scenario design and exploration, extensibility across all five elements of Multi-Domain Operations (MDO), and capability for human-human and human-AI collaborative reasoning and data collection, to aid development of AI reasoning and the warrior-machine-like interface. Here, we describe the ARL Battlespace testbed which fulfills the above requirements for AI development, training and evaluation. ARL Battlespace is offered as an open source software platform (https://github.com/USArmyResearchLab/ARL_Battlespace
). We present several example scenarios implemented in ARL Battlespace that illustrate different kinds of complex reasoning for AI development. We focus on 'gap' scenarios that simulate bridgehead and crossing tactics, and we highlight how they address key platform requirements including coordinated MDO actions, game theory and deception. We describe the process of reward shaping for these scenarios that will incentivize an agent to perform command and control (C2) tasks informed by human commanders' courses of action, as well as the key challenges that arise. The intuition presented will enable AI researchers to develop agents that will provide optimal policies for complex scenarios.