Dulce et decorum est (poem) - help with the yse, please?

It's about dissillussionment. If you've read a Farewell to Arms, it's a similar theme. The author is erting that, should readers see what he has seen, they would no longer romanticize or glorify death, war, or violence. The last lines, which he explicitly states is a lie, translate as "How sweet and proper it is to die for your native land", which is a quote by the Roman poet Horace and was commonly used like propaganda to boost morale. : )