What does walt whitman discuss in "o captain! my captain! " about abraham lincoln

Walt Whitman composed the poem "O Captain! My Captain!" after Abraham Lincoln's assassination in 1865. The poem is classified as an elegy or mourning poem, and was written to honor Abraham Lincoln, the 16th president of the United States. Walt Whitman was born in 1819 and died in 1892, and the American Civil War was the central event of his life. Whitman was a staunch Unionist during the Civil War. He was initially indifferent to Lincoln, but as the war pressed on Whitman came to love the president, though the two men never met.
Hope this helps!