Full Idea
The hiring of men to kill or be killed seems to mean using them as mere machines and insturments in the hands of someone else (the state), which cannot easily be reconciled with the rights of man in one's own person.
Gist of Idea
Hiring soldiers is to use them as instruments, ignoring their personal rights
Source
Immanuel Kant (Perpetual Peace [1795], 1.3)
Book Reference
Kant,Immanuel: 'Political Writings', ed/tr. Reiss,Hans [CUP 1996], p.9
A Reaction
Kant was not a pacificist, though this makes him sound like one. Some men go off to war with enthusiasm, and then regret it. Exploitation of rational beings may be the worst sin in Kant's Enlightenment world.