explain Who cheer when soldier lads march by,

Essay on The Great Gatsby, we are studying modern american literature

This is the prompt:

Consider the following passage from a World War I poem by Siegfried Sassoon:

You smug-faced crowds with kindling eye
Who cheer when soldier lads march by,
Sneak home and pray you\’ll never know
The hell where youth and laughter go.

Fitzgerald uses World War I as a backdrop for his novel. Both Nick Carraway and Jay Gatsby have all returned from Europe to start their lives again in America. What is the significance of war in The Great Gatsby?