In the General's speech about why he is sending Cage to the front lines, he mentions that after millions have died, the world will need someone to blame. Typically, everyone would blame the General (as he is the head of the UDF). His plan to save his reputation post-war is to have the face of the war (Cage) being filmed on the front lines as they win the war.
General Brigham: Operation Downfall, the entire might of the UDF
invading from France, the Mediterranean, Scandinavia, relieving
pressure in the Eastern front and allowing the Russians and Chinese to
push the enemy back and we all meet in the middle, exterminating this
Mimic scourge along the way. A lot of good soldiers are going to die
tomorrow, Major, and when the smoke clears and the body bags start
coming home people tend to look for someone to blame, someone like me.
Ideally I'd prefer a different scenario.
Cage: A best selling memoir perhaps? Maybe a career in politics? Off
the top of my head I would go with the..."The Sense of Manifest
Destiny", "Rags to Riches: Rags Rise Through The Ranks, Born to
Deliver". You see people, well, they'd love that sort of thing.
General Brigham: You misunderstand, Major. I didn't ask you here to
sell me, I want you to sell the invasion.
However, Cage was in advertising before the war, and is definitely not a soldier. He is not ashamed of this fact, and is rightfully terrified of being put into actual combat. On his way out, Cage tries to blackmail the general. His plan ruined, the general decides on revenge instead. He has Cage arrested and sent to processing at Heathrow, where the Master Sergeant is told that Cage is a deserter that tried to impersonate an officer.