The phrase "pity of war" is not used in this particular poem itself. Rather, it comes from the unfinished preface that Wilfred Owen wrote to an incomplete collection of poems that include Dulce Est Decorum. He wrote

This book is not about heroes. English Poetry is not yet fit to speak

of them. Nor is it about deeds or lands, nor anything about glory, honour,

dominion or power, except War.

Above all, this book is not concerned with Poetry.

The subject of it is War, and the pity of War.

The Poetry is in the pity . . .

Owen wrote these poems in order to evoke a strong emotional response from the reader. Unlike so much propaganda and English martial poetry of the past, he wanted to express the true horrors of the combat that he experienced. He wanted his readers to truly pity the senseless and horrific loss of life happening on the battlefront. That is what he means by the "pity of war." War is meant to be pitied, not glorified and celebrated.

There is a further meaning to this pity as well. Owen recognizes that war has long been a part of the human condition. This war is not the first and it will not be the last. Although there is a great potential for unlimited beauty in the world, it has become a pity that mankind reduces itself to murdering each other and inflicting the suffering that he depicts in his poems. This is the pity of war.

So even though Dulce Est Decorum does not actually use these words, they relate to its central theme. War and its ravages are something to be felt and understood for what they actually are. Glory is not achieved in the trenches, only pity for the unnamed young man who dies from poison gas and the countless others like him.