Government Documents including the military's are often free online, and part of public domain. This includes documents, working papers, guides and texts, etc.
Google allows you to search individual domains using the command: "site:"
Examples:
military biological weapons site:.mil
This asks Google to search "military biological weapons" at .mil, the Pentagon's domain.
military biological weapons site:.gov
This asks Google to search "military biological weapons" at .gov, the official government domain.
military biological weapons site:.dod.gov
This asks Google to search "military biological weapons" at DoD, the department of Defense.