In 2012 the Cornell physics professor and documentarian Robert H. Lieberman directed “They Call It Myanmar,” an impressive portrait of that country as it struggled to emerge from military rule. Mr. Lieberman’s latest, “Angkor Awakens: A Portrait of Cambodia,” scrutinizes another Southeast Asian country as it fitfully sheds the effects of a four-year national trauma: the Khmer Rouge.

The film wastes little time getting to that nightmare of genocidal self-destruction, but not before deftly covering the history preceding it: Cambodia’s prestige during the Angkor Empire in the ninth to 15th centuries; its years as a protectorate of France from the mid-19th century until its independence in 1953; a cultural flowering in the 1950s and early ’60s; and the impact of the United States’ bombing campaign during the Vietnam War. When a military coup in 1970 begot a government unfriendly to Communists, civil war ensued, presaging the ascendance of Pol Pot and the Communist Khmer Rouge in 1975.

And then, until 1979, came the horrors of a paranoid totalitarian dictatorship: the extermination of intellectuals; starvation; mass graves; the mountains of skulls made famous in the film “The Killing Fields.” An estimated two million perished. The graphic evidence here, in testimony on camera and in period photographs, is absolutely harrowing.

But “Angkor Awakens” concludes optimistically. Today, a young internet-savvy generation composes a majority. As that group deals with parents and grandparents still tormented by what one commentator calls “broken courage” — a form of post-traumatic stress — it offers hope to a badly scarred nation.