The original question had to be edited down. I would suggest that Paul's statement reflects the condition of being that he has come to accept as part of his being in war. There is a note of existential questioning within the statement that helps to bring out the sense of futility in terms of meaning about the nature of war. The carnival of death and destruction that has enveloped Paul has fundamentally changed him to an extent that is supersedes language and the ability to be articulated. In another point in the novel, Paul describes what his condition of being has become as a result of war:
I am young, I am twenty years old; yet I know nothing of life but despair, death, fear, and fatuous superficiality cast over an abyss of sorrow.
For Paul, it is this being that enables him to be able to define war with itself, something that cannot be fully articulated. In suggesting that "war is war," Paul suggests that his own being is a condition of life in which "it is what it is." There is sadness and inescapable sorrow, but it at a point in which Paul can no longer articulate its own condition. War has become its own reality, one that ensures a lack of hope in being for Paul.
I just read this book, All Quiet on the Western Front. This was a critical scene, so if you're doing this book in school or college read it carefully.
Paul despises the war, and I don't know if you read it yet, but he hates the sniper who whill get more awards if he kills more people. Then when he is in combat, he kills the French soldier, after I believe looking him in the eye. They did nothing to hurt one another, and rather it is their states that are fighting.
He is emotionally tormented, but justifies his act to calm his consience by saying "after all, war is war" because that's what their government tells them.