The Gift Outright by Robert Frost.  Kennedy gave Frost the option of reciting this poem or a new poem at the inauguration, and when Frost couldn't read his original poem, he recited this one from memory. My reading is of the 1942 original; for the inauguration, Frost changed the last line from "Such as she would become" to "Such as she will become." That change was made at Kennedy's request,  apparently to give the poem a greater sense of optimism.