The main difficulty in summarizing "On being asked for a War Poem" by William Butler Yeats is that the poem is so short. As it is only six lines long, your summary may need to be substantially longer than the poem itself.
The first piece of information you should include in the summary is the historical background. The poem was written in 1915 in answer to a request from Henry James that Yeats write a poem about World War I, which had begun in 1914, and in which the English were suffering massive casualties as they were exposed to the horrors of trench warfare.
Yeats responds by declining to write about the war, saying that giving advice to politicians isn't really the point of poetry, and then suggests that he writes not for political causes but for people such as old men and young women, who have leisure to read poetry for itself.
The poem can best be summarized as an argument against poets involving themselves in political issues, or serious matters of state, such as war. With the line "We have no gift to set a statesman straight," Yeats is simply implying that poets do not have the wisdom or authority to interfere in the business of politics, nor should they make uninformed statements about war and its causes. The key word in the poem is "meddling." "He has had enough of meddling..." The word's negative connotation speaks to the Yeats' less than favorable opinion of poets who involve themselves in politics. Yeats' concludes the poem by restating his opinion that poetry should be written to appeal only to those who have the leisure time to read and enjoy it, and not used as a platform for political change.