One of the primary messages of this poem is the responsibility the living owe to the dead. "The Dead" collectively speak in this poem and recall that just days earlier, they were vibrantly alive. They lived and loved, watching the natural splendor of sunset with their fellow soldiers.
Yet war has robbed them of life. They now lie underground below crosses, forever separated from the singing of the larks and other wonders of nature. They ask that those who are living "take up [their] quarrel with the foe." They have passed the torch of responsibility to those who are still alive to fight the battles that follow, asking that their efforts not be in vain. At first glance, it seems that the speakers ask that those who are living continue to wage war, fighting with the enemy as a means of retribution. Yet there also seems to be a consequence of all this death; after all, in asking the living to continue a war, the dead also ultimately ask for more people to die.
The message is thus complex. Honoring duty and country means that some soldiers will not return from the battlefield. While the living are called upon to continue the battles which have claimed lives, it is also important to remember that war is not glorious. Instead, the totality of its effects are horrific, cutting young lives short and robbing them of the experiences of the living.