The Panzer Soldier by Barry Sadler (Casca Series, Book 4) from Books In Motion.com

It was easy for Casca to go along with some of the basic premise behind the Nazi war machine. These men were soldiers, hard fighting men born of the same militaristic outlook that had sustained him since that fateful day at Calvary. Inevitably the high sounding ideals of the Third Reich revealed hidden horrors. In the background is always The Brotherhood. Discovering this hidden organization, and as the war begins going badly for the Nazis, Casca loses faith in the Fuhrer and other leaders. Finally, Casca has no choice but to visit the bunker in Berlin, a visit perhaps more important to the world than any other act he has ever committed.