A memorable exchange in Thursday’s Democratic debate concerned Henry Kissinger. For those who may be confused about the legacy of the 93-year old statesman and other political “ghosts from the past” who came up during the debate, here’s a quick guide.

“I am proud to say that Henry Kissinger is not my friend. I will not take advice from Henry Kissinger,” Vermont Senator Bernie Sanders thundered at his rival for the nomination, Hillary Clinton.

Responding to the former secretary of state’s praise for her predecessor, Sanders called Kissinger “one of the most destructive secretaries of state” in modern US history, while describing his actions in Southeast Asia as “one of the worst genocides in the history of the world.”

While Clinton struggled to explain why she valued Kissinger’s advice and friendship, many younger voters struggled to understand who Henry Kissinger even was – or why they should care. The hashtag #whoisHenryKissinger appeared on Twitter.

Some may be more familiar with Kissinger’s cartoon counterpart, “Dr. Henry Killinger,” a mysterious figure wielding a Magic Murder Bag on the TV show The Venture Brothers.

In what may seem like the distant past, between 1969 and 1977, the German-born Kissinger served as National Security Advisor, and then Secretary of State, for Presidents Richard Nixon and Gerald Ford.

During this period, the US extended the Vietnam War into nearby Laos and Cambodia, backing the murderous dictatorship of Pol Pot that killed millions. The US also backed a military coup in Chile in 1973, and the Indonesian occupation of East Timor in 1975. Both resulted in decades of bloody repression, claiming tens of thousands of lives.

For those asking #whoisHenryKissinger after last debate, here's a look at his war crimes: https://t.co/o7xXgNY4cUpic.twitter.com/ZNLtWqvLuy — The Intercept (@the_intercept) February 12, 2016

On the other hand, Kissinger helped President Nixon formulate the policy of “détente” with both the Soviet Union and the People’s Republic of China during the 1970s, leading to an easing of Cold War tensions. He is considered a leading proponent of “realist” foreign policy concerned with power rather than ideology. For example, while most US pundits were criticizing Russia’s intervention in Syria last fall, Kissinger advised letting Moscow defeat Islamic State.

“The destruction of ISIS is more urgent than the overthrow of Bashar Assad,” he wrote.

The Indispensables

“Kissinger is a friend, and I relied on his counsel when I served as secretary of state,” Hillary Clinton wrote in November of 2015 in her Washington Post review of Kissinger’s book World Order. “Though we have often seen the world and some of our challenges quite differently, and advocated different responses now and in the past, what comes through clearly in this new book is a conviction that we, and President Obama, share: a belief in the indispensability of continued American leadership in service of a just and liberal order.”

That phrasing would sound very familiar to the first female Secretary of State, who recently got in trouble with the Millennials for condemning women who wouldn’t back Clinton to a “special place in hell.”

Albright, Steinem slammed for 'shaming' women who don't back Clinton https://t.co/tT0G7V2ZTcpic.twitter.com/B2xHEFWYVW — RT America (@RT_America) February 7, 2016

Albright served in the first Clinton administration as the US Ambassador to the United Nations and went on to become Secretary of State during Bill Clinton’s second term. Before the “indispensable nation” quip, her most notable quote was from May of 1996, when she told Lesley Stahl of 60 Minutes that the deaths of half a million Iraqi children due to sanctions were “worth it.”.

There's a special place in Hell for Madeleine Albright. 500k+ dead Iraqi children: Worth it? #HillNo#WeAreBerniepic.twitter.com/fuvDx4Ntbq — Woobie Tuesday (@WoobieTuesday) February 7, 2016

Kissinger’s counterpart

However, the Czech-born Albright was not a disciple of Kissinger, but rather that of his most outspoken rival.

Zbigniew Kazimierz Brzezinski was the Democrats’ counterpart to Kissinger. He worked as a foreign policy adviser to President Lyndon Johnson in the 1960s and served as President Jimmy Carter’s National Security Advisor between 1976 and 1980.

A tireless Cold Warrior, Brzezinski despised Kissinger’s policy of détente. At one point, in July 1979, he championed sending aid to Islamist rebels in Afghanistan in order to provoke a Soviet invasion of the country and give “the USSR its Vietnam war.” Soviet forces entered Afghanistan in December of that year.

In a January 1998 interview with the French magazine Le Nouvel Observateur, Brzezinski explained that US support for the Islamists had preceded the Soviet intervention, rather than being a reaction to it, as official US history would have it.

“That secret operation was an excellent idea. It had the effect of drawing the Russians into the Afghan trap and you want me to regret it?” he told the interviewer. “What is more important in world history? The Taliban or the collapse of the Soviet empire? Some agitated Moslems or the liberation of Central Europe and the end of the cold war?”

Less than four years later, those “agitated Moslems” would destroy the World Trade Center and damage the Pentagon, setting off the endless “war on terror” that goes on to this day.