How do I write a thesis statement about Wilfred Owen's poem "Dulce Et Decorum Est"?

A good thesis statement needs to make a claim about the text, some statement that another person can argue with and that you can defend using evidence from the text. The poem itself describes many disturbing images that focus on the damage done to the young bodies of the soldiers—like how they become "bent double, like old beggars under sacks" or "knock-kneed, coughing like hags." Some have lost their boots and seem to wear boots made of blood because their unshod feet have become so bloodied as they march. Next, the speaker describes hearing gas shells dropping, likely chlorine gas, and watching another soldier fail to put on his gas mask in time. This man breathes in the gas, and it burns his lungs, causing them to fill with the blood that will eventually drown him. It is a slow, painful, agonizing death, and the men can do nothing but watch; there is no way to help him. The speaker finally cites a line from the Roman lyrical poet, Horace, which the speaker calls a lie: "Dulce et decorum est / Pro patria mori." This means "it is sweet and proper to die for one's country." After the horrible death that has been described, the way war bends and breaks the lithe bodies of young men, and the fact that it gives them nightmares forever, we can see why the speaker would not consider it "sweet" or "proper" to die in this way.
A good thesis statement, then, might read something like this:
In the poem, "Dulce et Decorum Est" by Wilfred Owen, the speaker says that it is a lie to suggest that it is sweet or proper to die for one's country because _______________, _______________, and _________________.
What three pieces of evidence could you use to defend this statement? How does Owen argue this?

You would first want to locate a major theme of the poem. For example, you might want to talk about "Dulce et Decorum Est" as an anti-war poem. Owen, having experienced World War I firsthand, shows in the poem the full reality of war's horror. He doesn't want to glorify or sugarcoat warfare in any way. He states that he wants to expose the sentiment "dulce et decorum est," which means in Latin "how sweet and right" war is, as a "Lie."
If you did choose to discuss this as an anti-war poem, you could write a thesis statement that sounds something like the following: Wilfred Owen wrote "Dulce et Decorum Est" to show that it is a lie to speak of war as "sweet and right"; instead, he reveals how terrible war really is.
Having your thesis statement in place, you would now pull evidence from the poem to support it. What images from the poem can you quote that show that war is horrible? There are many, so you should have no trouble.