CS Lewis is one of the most influential writers of the 20th century. His wise mind and linguistic skill have garnered a following of millions of readers around the world even today. As such, it is no surprise that many have wondered what his views on war were.
There is no shortage of places where we can find war in his writings and from which we can draw reasonable conclusions on what he thought about war. We will focus on just two examples, and briefly discuss certain passages and themes within them. However, first it is important to note Lewis’ background as a British soldier.
Lewis was conscripted as a 19-year old and stationed in France during WWI. He survived it with only a shrapnel wound to his chest, but the experience was nonetheless horrible.  He described it as memories of
the smashed men still moving like half-crushed beetles, the sitting or standing corpses, the landscape of sheer earth without a blade of grass, the boots worn day and night till they seemed to grow to your feet. 
He then served in WWII, but this time domestically as a Home Guard at Oxford.  How then did Lewis’ opinions, having experienced first-hand two World Wars, manifest themselves in his books?
About a third of the way through The Screwtape Letters, an experienced demon, Screwtape, mentors a newbie, Wormwood, on how to use a new, upcoming “European war”; Lewis meant WWII.
Consider whether we should make the patient [a young Christian man Wormwood wants in hell] an extreme patriot or an extreme pacifist. All extremes, are to be encouraged…Whichever side he adopts, your main task will be the same. Let him begin by treating the Patriotism or the Pacifism as a part of his religion. Then let him, under the influence of the partisan spirit, come to regard it as the most important part. Then quietly and gradually nurse him into the stage at which religion becomes merely part of the “cause” and his [faith] is valued chiefly for the excellent arguments it can produce in favour of the British war effort or of Pacifism.
It seems that Lewis saw that war was breaking out whether Christians would accept it or not, and the proper response was to still keep the faith’s values with utmost allegiance, yet join the fight for the cause of the country. Don’t let Patriotism consume to the point where fighting fueled with hatred and bloodlust, but don’t use religion to condemn those breaking the creed of Pacifism, either.
Another place war shows up in Lewis’ books is within The Chronicles of Narnia. All but The Magician’s Nephew and The Silver Chair have at least a couple of battles, after which Aslan usually appears to commend the various protagonists for their sacrifice and bravery on behalf of Narnia. In The Last Battle, Aslan even credits Emeth, a soldier from the enemy country Calormen, as being a seeker of good.
To those familiar with Narnia’s allegorical nature it may seem that Lewis is condoning war as some sort of test of allegiance to one’s country. Indeed, the land of Narnia, in its fight against neighbors who seek to subjugate it, represents the Kingdom of God on earth and the Christian’s fight to remain loyal to God and His commands whilst everywhere else society now ridicules the Bible and those who live by it. In The Screwtape Letters Lewis also writes that war is a very powerful reminder to all humans that death is coming, that the self is not omnipotent, and that focusing only on this life is foolish because of how short and uncertain it is. So did Lewis condone war over peace?
Lewis lived in a time probably all of us reading this can’t even imagine – a time where a dozen countries sent soldiers and bombs to fight and kill as many people as possible, twice! Based on the books of his that I’ve read, my conclusion is that Lewis, the author who came up with beloved fantasy worlds, was also a realist, and when and where he was born made it impossible for him not to see war as unavoidable, as being upon humanity whether or not anyone wants it to be. What he thought about how Christians should fight, however, is a whole other discussion.
You can find out a bit more on what Lewis thought of War in his essay “Why I’m not a Pacifist” published in The Weight of Glory and Compelling Reason.
Author’s note: I do know I’ve picked two examples of Lewis’ that happen to shine a pro-war light on him. There are surely works of his against fighting and against war, and if you know of any feel free to mention them below.