In an incident which has become notorious in philosophical circles, in 1956 a young philosopher by the name of Elizabeth Anscombe protested the awarding of an honorary degree by Oxford to former US President Harry S. Truman. “It is possible still to withdraw from this shameful business in some slight degree,” she wrote, “it is possible not to go to Encaenia [the award ceremony]; if it should be embarrassing to someone who would normally go to plead other business, he could take to his bed.” 
What provoked Anscombe’s protest was Truman’s authorization to use nuclear weapons at Hiroshima and Nagasaki. The bombings involved intentionally killing thousands of Japanese civilians as a means to concluding the war in the Pacific on favorable terms without a ground invasion of the Japanese home islands. Yet, says Anscombe, “choosing to kill the innocent as a means to your ends is always murder,” and as such is seriously wrong. She goes on to add, however, that “killing the innocent, even if you know as a matter of statistical certainty that the things you do involve it [such as attacking military targets when there are civilians nearby] is not necessarily murder.” 
This idea, that there is sometimes a morally significant difference between intentionally producing death (or other sorts of serious harm) and simply foreseeing it, is characteristic of the Principle of Double Effect (PDE), which is the topic of my book. The principle plays an important role in just war theory and international humanitarian law, but it is regarded with suspicion—or even outright hostility—by many contemporary moral philosophers. In my book I aim to push back by defending the principle from challenges and objections that have been raised to it in the years since Anscombe’s protest.
On basic question is this: Why think the distinction between what we do or bring about intentionally and what we do or bring about incidentally is ever morally significant? This is the grounding challenge. In response, I argue that the PDE can be grounded in a moral standard of solidarity, which measures our conduct in relation to others insofar as they are our fellow human beings. When one person intentionally harms another, the agent’s conduct manifests and expresses a desire to bring about harm for the victim, and this deviates from the standard of solidarity. Conduct that causes incidental harm also deviates from the standard when it is reckless, but not all incidental harm is recklessly produced.
The PDE faces many other significant challenges as well. For example, some critics claim that whether an agent does something intentionally, as opposed to incidentally, is a fact about her state of mind while acting, and they question why the moral permissibility of someone’s conduct should turn on what is going on in her mind. Others allege that the principle requires agents to “look inward” when engaging in moral deliberation in a way that is objectionable. And it has been claimed that recent empirical discoveries by cognitive scientists debunk the principle. In the book I respond to these challenges and others. My hope is that it will stimulate further debate about double effect.
 “Mr. Truman’s Degree,” pg. 71. In The Collected Papers of G.E.M. Anscombe, Volume 3: Ethics, Religion, and Politics, 62-71.Oxford: Basil Blackwell, 1981.
 Ibid., pg. 66.