"Dulce et Decorum Est" was written during World War I by a soldier who served in it, Wilfred Owen, and it harshly critiques that war. But the poem is also more generally a statement against all wars.
The phrase "dulce et decorum est" was a familiar line touted by wartime propaganda, suggesting that dying in the war in the service of one's country was the "sweet and proper" thing to do. This sort of patriotism was used to persuade thousands of young men to leave their homes and join the war effort.
Owen, in contrast, describes a modern battle scene that is anything but heroic. The soldiers are bedraggled, weary, and "bent over, like old beggars." A gas attack comes, and the poem's speaker graphically describes the frothing blood, burned lungs, and agonized "writhing" of the soldier who does not get his gas mask on in time. Owen depicts it as pointless suffering—bitter and meaningless, not sweet and proper. Nobody thinks of their homeland: they are simply trying to survive another day.
Owen ends the poem by making the message clear. His speaker tells readers that the idea that war is sweet and fitting is the "old lie." He also states that if people knew the reality of warfare they would not repeat this lie to young men "desperate" for "glory." By writing this poem, he is bringing civilians to the battlefield so they can witness the reality for themselves and start constructing a new narrative that discourages, rather than encourages, war.