"People ask me, 'Why you?' I come back and I say, 'Why not? Why just me? Where's everyone else at?" said Brett, who requested that ABC News not use his last name to protect his family back home.
"Jesus says, you know, 'What you do unto the least of them, you do unto me,'" he added. "I take that very seriously."
Brett returned to Iraq six months ago. In 2006-07, he served in the infamous "Triangle of Death," where he said he was badly wounded in an IED attack on his Humvee.
He came here, he told ABC News, to defend the defenseless -- to protect the Christians and others in this part of Iraq who have been terrorized by the ISIS onslaught: driven from their homes, massacred, their women and girls raped and sold into slavery.
He is of Irish and Polish descent, raised Roman Catholic and now describes himself simply as a Christian.
From Brett's forward position, in the deserted Christian village of Bakufa, about eight miles from the outskirts of the city of Mosul in northern Iraq, you can see the black flags of ISIS whipping against the sky in a stiff winter wind.
We joined him there to hear his story.