Dulce Et Decorum Est by one of the Lost WW1 poets, Wilfred Owen. This one has special significance to me because I first read it during Desert Shield back in 1990 when I was constantly having to don a protective mask in response to Iraqi gas strikes.
Herbsttag – Rainer Maria Rilke (Translations included). I can never find my favorite translation on the web, but the first of these comes close to how I see it.