Francis Lieber, the renowned German jurist whom President Abraham Lincoln recruited to formulate a comprehensive law of war during the U.S. Civil War, has stern things to say about assassination. The Lieber Code, with its innovative laws addressing the rights of prisoners of war, immunity for noncombatants, medical neutrality, and permissible weapons, had an enormous impact on the development of international law. Although the Geneva Conventions did not expressly adopt his unequivocal condemnation of assassination, his ban did make its way into the military manuals of many nations in language very close to Lieber's original. But what is Lieber talking about when he denounces a nation for slaying the enemy without a trial? After all, isn't this what soldiers do?

Fast-forward one hundred and fifty years. During the post–9/11 war on terror, the United States uses unmanned drones and specially trained teams to hunt guerrillas. In Yemen, there is spectacular success as intelligence successfully identifies the whereabouts of militants responsible for attacking the USS Cole in 2000 and killing 17 sailors.