“Peace upon earth!” was said. We sing it
And pay a million priests to bring it.
After two thousand years of mass
We've got as far as poison-gas.
Often Hardy’s end-rhymes feel like the massive stone door to a secret temple opening, but this one undercuts ‘mass’ with wickedly dark humour. 
 
 Chemical weapons, as we now call them, were first used during the First World War. In 1914 tear gas was used, a relatively ineffective gas which could literally only induce tears. However, the technology quickly advanced, and we soon had the most famous poison gas used in World War One,  Mustard Gas .The contemporary reader will probably be familiar with really evil stuff like  Sarin , and in fact, Hardy’s term ‘poison-gas’ sounds kind of naïve to modern ears– which proves further the accuracy and prescience of this poem. 
 
 As a military person will tell you, chemical weapons are extremely effective at indiscriminately killing innocent civilians, and extremely ineffective at winning battles, which is why they’re banned by the UN, and  why some advocated the United States striking Syria when they were deployed by the Syrian government in 2013. 
 Hardy frequently nails it when it comes to bleakness.
Christmas: 1924