War
Being a veteran of WWII, Jarrell wrote a great deal about the experience of war. Another well-known poet, Robert Fitzgerald said Jarrell was, "practically the only American poet able to cope with the Second Great War." So, if you come across a great poem about WWII, think Jarrell first—it's a pretty safe bet.
(Warning: If you thought you just found your new favorite poet because this guy writes, "wicked short poems!" you are going to be disappointed. Most of Jarrell's poems are normal poem length. Sorry.)