What is the thesis statement of "Dulce et Decorum Est"?

The thesis of this anti-war poem is that war is degrading and horrible. It is anything but "sweet and fitting" (which is what "dulce and decorum" means). People in English society might have been taught that it was sweet, fitting, and glorious to do one's patriotic duty by fighting to protect one's homeland, but Owen tries to illustrate that this sentiment is a lie.
Owen supports his thesis that war is terrible by using vivid imagery to illustrate how horrible and lacking in heroism modern warfare really is. Imagery is description that uses the five senses: touch, taste, smell, sound, and sight.
Owen's war setting is World War I, the war which was going on in his lifetime. World War I was a terrible bloodbath in which thousands and thousands of young lives were sacrificed in attempts to capture a few feet of territory. Specifically, Owen describes a soldier who suffers a poison gas attack.
Rather than opening with glorious, manly, upright fighters proudly engaging in battle, Owen's speaker, a solider himself, begins with imagery that compares soldiers to people often seen as weak and degraded in society: beggars and old women. He describes the soldiers as

Bent double, like old beggars under sacks, Knock-kneed, coughing like hags

This anything but a heroic image.
The harsh, degrading imagery continues. These soldiers are exhausted and injured:

Men marched asleep. Many had lost their bootsBut limped on, blood-shod. All went lame; all blind

A gas attacks adds to the misery. The men dive and fumble to put on their gas masks, but one doesn't get his on in time. There is nothing brave, manly, or heroic about his fate, which the speaker describes without trying to hide the horror:

the white eyes writhing in his face,
His hanging face, like a devil’s sick of sin;
If you could hear, at every jolt, the blood
Come gargling from the froth-corrupted lungs,
Obscene as cancer, bitter as the cud

The imagery of writhing eyes and frothing blood strips warfare of any heroism.
The speaker ends by directly addressing a "friend," who, we imagine, has been telling young people about the glories of war. He tells the friend that if he could see a real battlefield, he would no longer repeat the "lie" that war is noble and patriotic.
It is difficult, once one has read this poem, to understand modern warfare as anything but a horror to be avoided.