No / Set in the context of efforts to utilise chemicals as weapons of war, that have their origins in collaborative efforts between the UK and the US during World War I, this chapter examines the origins, the evolution, and the hostile misuse of chemical anti-crop agents and defoliants. Out of efforts between the two countries that endured throughout World War II, military interest in chemical anti-crop agents and defoliants emerged in-part as a consequence of a close association between civilian chemistry and military chemistry. It is shown by way of insights provided from official sources from the United Kingdom (UK) National Archive that UK use of such agents in Malaya resulted in the emergence of new techniques concerning the large-scale use of chemical anti-plant agents, as well as methods for their widespread dissemination. It is argued here that the above can be seen as a prelude to subsequent use in Vietnam, the latter having implications of relevance to human health and for the environment. It is shown that the role of science policy experts in bringing influence to bear on policy-makers during the latter part of the Vietnam War was significant in bringing about change in policy and an end to use in Vietnam. Also of significance is the issue of chemical weapons in the context of efforts to codify the norm of non-use under the Geneva Protocol, under the Chemical Weapons Convention (CWC), and under a range of related prohibition regimes. This chapter considers the implications of the scientific and technological developments in phytobiology of relevance, in particular, to auxins (work on endogenous growth regulators—auxins—would lead to the discovery of “the first systemic or hormone herbicides”). The findings are drawn together in a concluding section at the end of this chapter.