Apologies to the moderators, as I'm reasonably certain that I am breaking the 20 year rule here. Depends on how you count it. The acts described took place in the mid-90's up until 2003. The trial was in 2006-08, with the appeal continuing to 2010 and post-conviction proceedings well into 2012.

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Charles Taylor was the President of Liberia from 1997-2003. Before that he ran a rebel group that fought for decades in Liberia. But he's not really the star of the story here. In 1977, while attending college at Bentley University in Boston, he fathered a son, colloquially known as "Chuckie Taylor." Born as Charles McArthur Emmanuel, and changed his name legally to Roy Belfast, Jr. I mention this because he is an American citizen by birth.

In 2008, he was convicted of the crime of "torture" under United States laws (18 USC § 2340A) among other crimes, and sentenced to 97 years in prison. The substance of his conviction was running the Anti-Terrorist Unit or ATU of the Liberian state security apparatus, which ran several prison and forced labor camps, as well as, to put it lightly, death camps in which people were tortured, questioned, and publicly executed.

My sources come from the publicly available documents and pleadings, which can be found at United States v. Belfast, 06-cr-20758 (S.D. Fla. 2006). There is also the publicly available appeals court decision here: United States v. Belfast, 611 F.3d 783 (11th Cir. 2010), which begins with the rather memorable recitation of facts: "The facts of this case are riddled with extraordinary cruelty and evil."

Doesn't strictly meet your criteria because he wasn't tried, but Maj. Gen. Curtis E. LeMay said he would have expected to be tried for war crimes for the bombing of Japanese cities had the allies lost WW2.