A World War II hero returns to Germany to solve a mystery -- and meet an enemy

Updated 0027 GMT (0827 HKT) November 11, 2018

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(CNN)Clarence Smoyer's index finger caressed the trigger. Sweat poured down the leather flaps of his helmet. No one in his tank moved or even whispered.

It was March 6, 1945, and Smoyer was part of the Allies' last push into Nazi Germany. The lanky 19-year-old with a mop of curly hair was part of a tank crew that had crawled into the German city of Cologne for what would become the US Army's biggest house-to-house fight in Europe. The Germans called it "Endkampf," the final battle for their homeland.
"Gentlemen, I give you Cologne," Smoyer's commander announced over the radio. "Let's knock the hell out of it!"
Smoyer didn't need any added motivation. Before he entered the shattered city, he'd received word that his cousin and his wife's brother had both been killed in the war. Those bastards are going to pay, he vowed.
Clarence Smoyer during the war. He gained a reputation for deadly accuracy as a tank gunner.
Now he intended to fulfill his promise. His M26 Pershing tank had just been engaged in a shootout with a German tank at a sprawling intersection in the town's center. But then the enemy tank ducked behind a building. Smoyer searched for it, scanning a hellish urban landscape of rubble, sagging streetcar cables and collapsed buildings.
"Staff car!" someone yelled over the radio.
    A black Opel streaked into the intersection. With orders to shoot anything that moved, Smoyer pressed the trigger. Bullets and tracers from Smoyer's gun smashed into the car; ordnance from another source also flew through the intersection. The car crashed into the sidewalk, and then Smoyer saw something that made the pit of his stomach fall out.
    The car's passenger door