The theme is primarily the wastefullness and pointlessness of war in the sense that it achieves nothing more than the tragic loss of life. The tone is mournful as the persona recounts his most vivid memories of war. It is also, at times, slightly ironic as the persona repeats the phrase "dulce et decorum est pro patria et mori" not because he believes it but because he wants to emphasise how ridiculously incorrect it was and how it was used irresponsibly to encourage men to enlist.
The rhythm matches the activity the men in the poem are doing at the time. When they are marching, the rhythm reflects this. Hwowever, when the gas attack occurs, the truncated sentences create a much quicker pace, reflecting the panic of the event.
The most effective images is a bit more subjective. Personally I would say it is the image of the man dying from the gas attack. The use of inclusive language makes this image quite vivid to the responder. This is supported by the use of similes "like a man in fire or lime" and "as under a green sea" which help the responder to visualise this harrowing event. Owen's description is also emotive, using words such as "drowning" or "helpless" to indicate how the persona is feeling about the event. This is also the image that conveys a lack of real concern for the soldiers, as shown by the use of the word "flung." The other idea that makes this image powerful is that it is the image that haunts the persona, as demonstrated through the hyperbole, "in all my dreams". It has clearly been a signficant image for him and is recounted in such vivid detail that it becomes a significant image for the responder also.