Alistair MacLean served in the Royal Navy during World War II and was stationed in the Aegean Sea for part of it, participating in the bombardment of Milos and also sinking blockade runners around Crete. So when he's writing about military operations in the area, his words contain the realism of someone who's actually experienced some of these things.
The book is also an examination of what Wilfred Owen termed, "...War, and the pity of War." While an adventure story, Guns is not a mindless glorification of battle. It very clearly portrays the cost of war, on both a personal and societal level: the devastation and death, the suffering and trauma- both physical and mental- it causes.
MacLean wisely doesn't make all of the Nazis cardboard cut-out baddies; they are ordinary men who happen to be fighting for a side which is doing extraordinarily evil and depraved things. Which, in a way, is more horrific to contemplate than if they were all moustache-twirling villains. One scene in the novel which is particularly memorable is when the team is temporarily pinned down but hold the high ground. The German commander orders his men up the hill even though they will be sitting ducks, out in the open on the hillside, above them men with machine guns and cover. The German soldiers, knowing they're heading into almost certain death, obey. Mallory and the others, sickened at the thought of slaughtering men who can't defend themselves nevertheless do it, because the alternative is the failure of their mission and the deaths of perhaps thousands of Allied soldiers on Kheros. This is not a glorious deed to be celebrated, but a grim and terrible one which is done because it has to be done. The pity of war.
Guns also, in the characters of Andrea and the other resistance fighters, gives a glimpse of something which doesn't get talked about a lot when discussing W.W. II- the suffering which the Greeks endured at the hands of the Germans and their allies. They perpetrated a number of massacres of civilians, and sent all of the Greek Jews to concentration camps. It is estimated that there were over 400,000 Greek casualties during the German occupation, and the entire Jewish population was exterminated. One of the many horrors perpetrated by the Nazis, and why it was so necessary to stop them whatever the cost.