It's actually pretty conventional to label any research that involves changing the environment of people as involving "human subjects" -- even if the purpose of the research is not directed at understanding how the change affects people.
Here [0], for example, is a U.S. Department of Energy web page that defines the "scope of research" for human subjects studies. It says, in part, [R]esearch using human subjects encompasses a broader range of research than many investigators... often realize. In addition to traditional biomedical and clinical studies, human subjects research includes, but is not limited to, studies that... evaluate environmental alterations
To make that even more explicit, they have a "regulations" web page [1]. The top link there is to a memo that clarifies that any "research involving intentional modification of an individual's or a group of individuals' environment" must be managed according to the rules for human subjects research.
(All that said, it's possible that the military did go through an internal "human subjects" review, which might have concluded that the trials were OK, on the basis that the agents were supposed to be harmless.)
[0] http://humansubjects.energy.gov/research/scope.htm [1] http://humansubjects.energy.gov/regulations/