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'Succession' star Brian Cox is 'relieved' to learn this ancestry discovery: 'This is major'

The actor hears his grandfather's untold story in an episode of "Finding Your Roots."

"Succession" star Brian Cox gains new respect for the grandfather he never met in the Jan. 31 episode of "Finding Your Roots."

While discussing his family history with show host Henry Louis Gates Jr., the actor explains that his mother's father, James McCann, had a troubled life and died relatively young.

Cox's mother had always told him that McCann served in the military, but whenever the actor attempted to research his ancestry, he would run into dead ends.

During the episode, Gates confirms that McCann was indeed in the military and worked as a sergeant.

“Oh my God!” Cox, 76, exclaims as he hears the news.

By researching pension documents for McCann, the "Finding Your Roots" team discovered that the veteran was a part of the Black Watch, an infantry battalion of the Royal Regiment of Scotland also known as the Royal Highlanders, at the height of World War I.

During the war, McCann likely saw lots of harsh combat on the frontlines of the war. He was also hospitalized for what was likely gas poisoning.

After coming home, McCann struggled with alcohol dependency, according to Cox. Once he heard about the horrors his grandfather likely endured on the battlefield, the actor says he has a greater understanding of McCann's behavior.

"I think that explains a lot. It's not a fantasy. It's a reality and he lived with that harsh reality," he says.

The 76-year-old says it "makes sense" why McCann, who died at the age of 50, struggled after he came home from the front.

"It doesn't excuse anything, but it fills in a lot about what he was going through, and what he went through and how he didn't deal with it, how he couldn't cope with it," he says.

“It’s relieving because it’s like a ghost has been laid (to rest) finally," he says. "He had a particularly bad (reputation). He was a drunk. And nobody's taken into account any of this."

McCann went on to receive four honors for his military service. Cox gets emotional while hearing the news, calling it "incredibly moving."

"This validates his life. His life was pretty (invalidated) up until this point, and this is a major, major, major thing for him. I'm so, so grateful for this, I can't tell you," he says.