This is one of Hardy's many poems concerning war and its various absurdities and tragedies. Here, the poem, told in simple language with an absence of figurative language, narrates to us how "quaint and curious" war is through discussing an incident told from a common soldier's point of view when he shot an enemy just because he was his enemy, when if they had met in any other situation they probably would have had a drink together. Note how the poem begins:

Had he and I but met
By some old ancient inn,
We should have set us down to wet
Right many a nipperkin!

However, the one thing that prevents this from happening is that they meet each other in war, and on the opposite sides of war. Thus the soldier is left perplexed by what war does to us and how in war acts of violence and murder are justified, when elsewhere, the same man that he killed he would have helped:

Yes; quaint and curious war is!
You shoot a fellow down
You'd treat, if met where any bar is,
Or help to half a crown.

War is such an absurd situation as it forces us to go against a natural, human, good nature and kill those we, in any other situation, would have been kind to.