Title: Probe Into Multi-agent Adversarial Reinforcement Learning through Mean-Field Optimal Control. Abstract: Multi-agent adversarial reinforcement learning (MaARL) has shown promise in solving adversarial games. However, the theoretical tools for MaARL's analysis is still elusive. In this paper, we take the first step to theoretically understanding MaARL through mean-field optimal control. Specifically, we model MaARL as a mean-field quantitative differential game between two dynamical systems with implicit terminal constraints. Based on the game, we respectively study the optimal solution and the generalization of the fore-mentioned game. First of all, a two-sided extremism principle (TSEP) is then established as a necessary condition for the optimal solution of the game. We further show that TSEP is also sufficient given that the terminal time is sufficiently small. Secondly, based on the TSEP, a generalization bound for MaARL is proposed. The bound does not explicitly rely on the dimensions, norms, or other capacity measures of the model, which are usually prohibitively large in deep learning.