To understand the allusion used in Carl Sandburg's poem "Grass" one must understand what an allusion is.
According to the eNotes site, an allusion is
a reference, usually brief, often casual, occasionally indirect, to a person , event, or condition thought to be familiar (but sometimes actually obscure or unknown) to the reader.
Therefore, allusions are used to allow the poet to make reference to something in regard to provide a deeper understanding of the work.
In Sandburg's poem "Grass," the allusion made functions indirectly. The grass, while depicted as the object, is not really grass. Instead, it refers to the people who needed to cover up the bodies of those lost in the battles which took place in Austerliz, Waterloo, Gettysburg, Ypres, and Verdun. The grass and the people involved in the battles needed to cover up the bodies so that they would be forgotten. The poem illuminates this when the reader comes to the following lines:
Two years, ten years, and passengers ask the conductor:
What place is this?
Where are we now?
The grass needs to work (personification) so as to cover up the bodies of those lost in battle/war. The grass, like some involved in the battles, wish the bodies to be covered so as to make the world forget that the lives were lost or taken.
In the end, the grass is not moved by the lives lost in battle. Instead, the grass simply states: "Let me work." This shows the grass, and those responsible for covering up the lost lives, as being emotionless when it comes to death. Neither the grass nor those wishing the bodies to be forgotten felt compassion for the lost lives. Instead, the bodies would simply be covered up...and forgotten.