Walt Whitman's "O Captain, My Captain" is one of America's classics.  A beautiful, deep, and exquisite piece that shall influence America forever.  Whitman wrote this poem around 1967 and it was included in his 1967 edition of Leaves of Grass.  He wrote it at the end of the civil war and put heart into the piece, which made it sincere and genuine.
     Whitman's poem is all about how President Abraham Lincoln led America through the Civil War and back out again.  The poem describes how important Lincoln was by calling him the captain and father of the ship.  Of course, the ship represented the American people, who would have been at loss without their lighthouse beacon-Abraham Lincoln.  The poem begins by saying how exciting it is that the nation (ship) weather'd every rack, overcome every obstacle, and won the the thing we were seeking: peace, unity, and freedom.  Everyone is rejoicing in the good fortune, and congratulating Lincoln for getting them (ship) through all this devastation.  To their dismay, Lincoln isn't there, he has fallen.  He led America (ship) through the horrifying seas of war, but at the end he, himself, is taken-assassinated.  Lincoln was assassinated by John Wilkes Booth on April 19, 1865.  In the poem the American people want Lincoln, ("for you (Lincoln) bouquets and ribbon'd wreathes, for you the shores a-crowding") but he's gone, Lincoln anchored the Americans safe and sound, and they all are ecstatic over what he's done.  But a shadow is cast over the celebration, as their captain and father, Abraham Lincoln, has fallen, cold, and dead.

    For more information on the main points of "O Captain, My Captain":
    Abraham Lincoln
    Civil War