Du Pont was a French soldier during World War II. He was abducted by the War Lords and hypnotised into fighting in their simulated version of the war.
When the resistance took over the chateau and killed Smythe, the War Lords decided to use several groups of soldiers to attack the chateau. Du Pont was one of the soldiers, and when he threw a grenade in one of the windows, he was dragged through it by Lieutenant Jeremy Carstairs. The Second Doctor then used the processing machine that he stole from the War Lord HQ to de-process Du Pont. Du Pont recovered and was taken by Carstairs, who told him what was going on.
Like all surviving victims of the War Lords, he was returned home by the Time Lords. (TV: The War Games)
Behind the scenes
Du Pont is never referred to by name in the dialogue; his name is only given in the closing credits.