Drum Taps by Whitman

In the poet "Drum Taps", Whitman describes preparations for a war ("The young men falling in and arming", "Outdoors arming—indoors arming—the flash of the musket-barrels").
In the abstracts he describes the terrible feeling he had while he was serving probably serving as a nurse.
("To sit by the wounded and soothe them, or silently watch the dead.")
All these events take place at Manhattan, New York City. So you can assume that Whitman has served for the Union. In this poet, he emphasizes how sad and terrible he feels about this war.