Howard Zinn died this year. He is perhaps best known for his People's History of the United States, a book that has featured in The Simpsons and was recommended by Matt Damon's character in the film Good Will Hunting. This book, which offered a view of US history in terms of 500 years of imperialism, colonisation and racism, was less well received academically, with critics calling it polemical and revisionist. Zinn ultimately was an activist and it shone through his academic work as well as his more political essays.

Delivered to the publisher one month before his death, The Bomb falls into the latter category. In it, Zinn puts two essays side by side, one entitled "Hiroshima, breaking the silence", the other "The bombing of Royan". As a young man eager to be demobbed, Zinn recalls celebrating the dropping of the atomic bomb; it meant the end of a war he did not wish to return to. He had taken part in the bombing of the French town of Royan just three months earlier. The essays revisit that unthinking celebration and desire to follow orders of those months in 1945. Using historical evidence, it also argues that neither mission was necessary and asks what prompted military action that would transcended military logic and moral sensibilities.

Like Zinn, I have changed my mind over the need and glory of war. Leaving Quaker school at 17, I wanted to be a fighter pilot. But travelling the world on my bicycle, I came to the same realisation as Zinn – that there is no "them", but only a global "us". I will gladly say that changing one's mind is not and should not be seen as a sign of weakness, as it so often is for politicians, but of creative reflection. Of course, now that I am a committed pacifist, I hope the changes people make follow the same direction as Zinn and me rather than the other way round – from pacifist to militarist.

However, Zinn is also involved in arguments more complex than a simple pacifist one. He is critical of portrayals of any portion of humanity as "lesser" and rightly points out that only by dehumanising the enemy could strategies such as blanket bombing or the dropping of atomic bombs be perceived as possible by people who also saw themselves as moral. I remember an analysis of the media by the sociologist Christie Davies which explained how humanity could at any point be counted as identified humans, nameless members of a group or statistics, and that their moral status shifted within press coverage depending on the degree of humanity ascribed to them. "Eighteen die in bus crash" constructs the dead as a statistic. So it is with war, where "the enemy" is dehumanised or even demonised to the point where killing them is not perceived as murder, and where there are no longer "innocent" victims, just "dead enemies".

This is a conscious process of state and media which can be seen in the censorship of films documenting the effects of the atomic bombs in the years following the war. Zinn implicitly argues that if we place ourselves into that "enemy" situation and cannot justify the military action proposed, then we are morally at fault. This may end up as a kind of pacifism, but it is one which takes critics on in different ways and asks more pointedly for each proposed action to be examined in a globalising moral light.

In these particular cases – especially the destruction of Royan, which was actually inhabited by allies rather than enemies, Zinn argues that motives of military pride, experimentation of new technology (napalm was used for the first time at Royan) and the desire for revenge outweighed the facts that none of it was strategically necessary – the port was a sideshow which posed no threat to the rapid advance of the allies towards Berlin in June 1945.

That said, the very "evils" that the war was meant to defeat was implicit in the actions of the allies. All of the allied powers had records of colonisation and all had previously invaded other countries for their own good, as they then complained of Germany or Japan doing. All defended their empires against independence movements in the years following 1945. All ultimately carried out military action that killed thousands and thousands of civilians. Blanket bombing in Dresden was described by Churchill as a "heavy raid". At the time, racism in the US underpinned the social system as much as it fuelled the rhetoric to go to war against Japan and Germany. In this sense too, less happily, "they" were actually just like "us". Yet, the rhetoric of war relies on "them" being seen as lesser.

The Bomb is not an easy book to read in places, given the accounts of the suffering inflicted by the bombings. It is one that will infuriate many. Some will resist its historical analysis, some its collage of arguments in its favour, and some will say Zinn just didn't understand the true nature of the decisions that had to be (and still are) made. What he shows however, is the divide between those in the corridors of power, and those of us who do not really know what is going on and only have their polemic of the necessity of war to go on.

Unfortunately, Zinn's book remains timely and crucial. As a last testimony to a life of scholarship and activism, it serves us well to take his writing seriously.