The destruction of stockpiles or unexploded ammunitions of nitrogen mustard (tris (2-chloroethyl) amine, HN-3) requires the development of safe processes. The thermal destruction of this kind of compound is one of the most efficient method of destruction. Because of the high-level of toxicity of this chemical, there is a considerable lack of knowledge on the chemical kinetics of this toxic at high temperatures. In this study, a detailed chemical kinetic model for the pyrolysis of nitrogen mustard gas is developed based on a large number of thermo-kinetic parameters calculated with theoretical chemistry. The thermal decomposition of HN-3 is shown to mainly proceed through stepwise dechlorination making with Cl-atom being the principal chain carrier. The successive losses of chlorine atom mainly lead to unsaturated amines without chlorine groups. Theoretical calculations demonstrated that the thermal decomposition of these compounds ultimately lead to the formation of pyrrole, which can accumulate at low temperature. At higher temperatures, pyrrole yields HCN and acetylene. Simulations also predict that about 52% of the total flux of decomposition of HN-3 leads to the formation of N,N-diethenyl-2-chloroethylamine (P29) which acts as a chain branching agent because its unimolecular decomposition is preponderant and products one chlorine and one hydrogen atoms. Comparisons with the simulated reactivity of sulfur mustard gas are also performed and show that HN-3 is more reactive that the former toxic. The higher number of chlorine atoms in HN-3 compared to sulfur mustard (3 vs. 2) and the formation of the chain branching intermediate P29 during its decomposition explain this behavior.