Wilfred Owen is suggesting not so much that war itself is a lie, but that those people back home, particularly politicians, who claim that it is a glorious thing to fight and die for one's country, are liars. The title of the poem refers to a poem in Latin by Horace, who said, Dulce et decorum est pro patria mori (How sweet and fitting it is to die for one's country.)
To demonstrate that this sentiment is, in fact, based on a lie, Owen juxtaposes it with the brutal reality of war, both the misery of soldiers trudging along, and the horrific final struggles of a young man who cannot get his mask on quickly enough to survive a gas attack:
Gas! Gas! Quick, boys! – An ecstasy of fumbling,
Fitting the clumsy helmets just in time;
But someone still was yelling out and stumbling,
And flound'ring like a man in fire or lime...
Dim, through the misty panes and thick green light,
As under a green sea, I saw him drowning.
In all my dreams, before my helpless sight,
He plunges at me, guttering, choking, drowning.
If the people back home could see these horrible scenes, Owen says, perhaps they would not repeat Horace's old lie.