The poem "I Have a Rendezvous with Death" has, at its central theme, the inevitability of death particularly when one has given one's life in the pursuit of something that can only be obtained by a personal sacrifice. The poem, which is told by a WWI soldier's point of view (Alan Seeger was an enlisted soldier in the Foreign Legion , 1914, prior to the USA entering the conflict), allows us to see the world as he would have seen it. 

World War I was something never seen before in history. They called it "The Great War" because of so many countries (allies) were involved against the enemy. The war changed every social construct seen prior to 1914, ranging from class, to honor, and everything in between. WWI also brought with it the first victims of "shell-shock", or what we know today as Post Traumatic Stress Disorder, or PTSD. 

The soldier in this poem has seen all of this, while the rest of the world has not. However, he brings scenes of idealism where he actually understands that death is a real consequence in his case, so he must accept it and even welcome it.

But I’ve a rendezvous with Death
At midnight in some flaming town,   
When Spring trips north again this year,   
And I to my pledged word am true,   
I shall not fail that rendezvous.

For this reason, the poem is also an elegy. There is a lot more to study about Seeger, and his life is a very interesting one. Those of us connected to the military are awed by the sense of patriotism in the light of death and war that Seeger exhibited throughout his life. His is not a sad story of war; he actually seems to have embraced his role as a soldier in a big conflict. The constant idealism and literary licenses that he applies to the poem makes it read more condescending and tender than tragic. It seems contradictory, but through the eye of this poet in particular, the poem ends up making sense.