Federated Learning (FL) is increasingly being adopted in military collaborations to develop Large Language Models (LLMs) while preserving data sovereignty. However, prompt injection attacks-malicious manipulations of input prompts-pose new threats that may undermine operational security, disrupt decision-making, and erode trust among allies. This perspective paper highlights four vulnerabilities in federated military LLMs: secret data leakage, free-rider exploitation, system disruption, and misinformation spread. To address these risks, we propose a human-AI collaborative framework with both technical and policy countermeasures. On the technical side, our framework uses red/blue team wargaming and quality assurance to detect and mitigate adversarial behaviors of shared LLM weights. On the policy side, it promotes joint AI-human policy development and verification of security protocols.

Federated Learning (FL) allows multiple clients to collaboratively train shared models without exchanging raw data, thereby preserving privacy. However, FL systems are vulnerable to malicious participants known as free-riders who exploit the collaborative nature without providing genuine data contributions. To expose this critical security threat, we introduce a novel stealth free-rider attack that leverages pre-trained forecasting models to generate highly realistic synthetic time-series data. Our approach enables malicious clients to deceive FL systems while obtaining benefits from fair participants' contributions, thereby undermining the integrity of federated networks. Numerical results on EEG-based sleep stage classification demonstrate that our attack maintains comparable performance with free-rider ratios up to 70% while causing catastrophic degradation when all clients are free-riders.