Why Gatsby's life had been confused because of the war?
Jay Gatsby's greatest ambition in life is to woo Daisy. So, prior to the war he is well on his way to doing so but problems arise because of his relatively humble background as well as the fact that he is called off to Europe.
So in the years he is gone, Daisy finds someone who can provide her the life she feels she needs, in this case Tom Buchanon, and marries despite perhaps never having felt for him the things that she does for Gatsby.
This brings considerable difficulty to Gatsby's efforts to get Daisy to be with him, particularly now that she has a history and a daughter with Tom and Gatsby's fabulous wealth is a little bit suspect.