"Grass" mentions a number of famous battlefields across Europe—Austerlitz, Waterloo, Ypres, and Verdun—as well as Gettysburg, right in the great state of Pennsylvania. The thing that these once-bloody battlefields have in common is that they no longer look like they did during war time. Green grass has grown over the hills; Gettysburg today looks even serene. (Don't believe us? Check out these photos.)
Time, and the grass, have erased the ravages of war from the landscape—the dead bodies, the injured soldiers, the guns, the horses. All we've got now is grass, grass, and more grass. In that way, the physical setting of this poem is subject to inevitable repair (the unstoppable grass sees to that). But the speaker seems to be reminding us about the emotional importance that this setting should convey. A lush green landscape may hide the horrors of our wartime past, but it's up to us to remember what the landscape can't.