This poem has a sad tone because even though the war is won, their President is dead making the people of America very sad in a time they should be celebrating victory.
"But O heart! heart! Heart! " This is an alliteration because it starts with the same letter and sound.
This arm beneath your head!
The first four lines of the author's eight line stanza would always tell of how everyone was celebrating the captain and his achievements, but the last four lines would always talk about how the captain is dead.
The theme of the poem is that even though the ship (America) has won the war, its crew (the American people) will not be able to celebrate because their captain (President Lincoln) is dead, giving the poem a sad tone.
The extended metaphor is that America is being called a ship with president Lincoln as its captain.