People who fled from the violence in Mosul walk inside the Khazer refugee camp on the outskirts of the Kurdish city of Irbil, July 17, 2014. REUTERS

As he made a statement on the Iraq crisis and U.S. decision to militarily intervene on Thursday, President Obama made two references to "genocide."

There are few words that cause more horrifying memories and provoke stronger reactions. Politicians usually use "genocide" with caution. Obama, however, was unequivocal: "[Islamic State] forces have called for the systematic destruction of the entire Yazidi people, which would constitute genocide." He went on to say: "We can act, carefully and responsibly, to prevent a potential act of genocide."

At the beginning of this week, the Yazidis were unknown in many parts of the world. Now, they they appear to have redefined Obama's Iraq policy. The term "genocide" seems to have changed everything. How could one word have such power?

'Genocide' and its origins

"Genocide" became a common notion at the end of World War II which explains a lot about the nature of the word itself: The atrocities committed by the Nazis had created the necessity to find a new word capable of describing horrors of previously unknown scale. "Genocide" is composed of two words: "genos" (which is Greek and means race) and "cide" (which can be translated as: to kill). It found its first notable mention in the 1945 military tribunal in Germany which convicted top Nazi criminals. Shortly afterward, in 1946, genocide was officially banned under international law by the United Nations, which had been established as an organization a year earlier.

The United Nations defines it as:

... any of the following acts committed with intent to destroy, in whole or in part, a national, ethnical, racial or religious group, as such:

(a) Killing members of the group;

(b) Causing serious bodily or mental harm to members of the group;

(c) Deliberately inflicting on the group conditions of life calculated to bring about its physical destruction in whole or in part;

(d) Imposing measures intended to prevent births within the group;

(e) Forcibly transferring children of the group to another group.

Under the 1948 Genocide Convention, the U.N. requires its members to prevent and punish acts of genocides.

Genocides occurred before the word for it was even coined. In 1915, during World War I, approximately 1,5 million Armenians were killed by Turks as the decline of the Ottoman Empire progressed. Earlier this year, current Turkish Prime Minister Recep Tayyip Erdogan expressed condolence for the first time -- at least partially -- for the killings that happened almost a century ago. The Armenian killings are generally considered to be the first genocide in recent history.

A troubled history

Genocides may be planned long in advance, but they often move very fast once they begin. When the international community takes notice the killings are already underway. For example, between 1992 and 1995, some 100,000 people were killed in Bosnia-Herzegovina. It took NATO two years to intervene.

Close to a million people are on their way back from Zaire to Rwanda, 1996. They had taken refuge in Zaire following the genocide of 1994. REUTERS/Peter Andrews

Around that time, an even larger genocide emerged in Africa which was initially dismissed as a civil war. After just 100 days about 800,000 Rwandans (most of them belonging to the Tutsi minority) had lost their lives while the international community was unsure of how to respond. U.N. peacekeepers in the country when the fighting began were actually forced to withdraw as violence spread. The United States was one nation that avoided military intervention in the genocide, in part due to the recent "Black Hawk Down" incident in Somalia, in which a U.S. helicopter was brought down by militants. The Rwanda killings were only recognized as genocide after the fact.

These two tragedies were key in pushing the international community to take genocide prevention seriously. However, even after learning the lessons from these two infamous events, the word "genocide" can still be fraught. It took months of investigation and discussion within the State Department before Secretary of State Colin Powell would say in 2004 the killings in the Darfur region of Sudan amounted to genocide. Even then, the Sudanese government and others argued that the United States should not have labeled it so before an inquiry.

A balancing act

Last year, Alex de Waal, the executive director of the World Peace Foundation, warned that the mislabeling of genocide to any situation of ethnic tensions and governmental breakdown will make it "lose its analytic power and its special moral force." In other words: It needs to be used with caution.

The current catastrophe on the peaks of Mount Sinjar where 40,000 mostly Kurdish-speaking Yazidis are certainly trapped has characteristics of genocide. According to the U.N., one of the indications is that the killings are directed against one group with the aim "to bring about its physical destruction." Islamic State, the al-Qaeda-inspired group that is advancing toward the Kurdish city of Irbil, provoking the current U.S. airstrikes, has justified its planned slaughter by claiming that the Yazidis are "devil-worshipers." The Yazidis are unable to defend themselves. Being trapped on a mountain, the targeted group would be either doomed to die from hunger and thirst or execution by Islamic State militants.

In this image released by The White House, President Obama meets with the National Security Council in the Situation Room of the White House on Aug. 7, 2014. (AP Photo/The White House, Pete Souza)

The United States has the capabilities to intervene in this rapidly escalating situation and it was asked to do so by Iraq's government. Plus, the United States is a signatory to the 1948 Genocide Convention, and is thus committed to preventing and punishing genocide.

By uttering the word "genocide" Obama provided the moral weight needed to support another military intervention in Iraq. Polls show intervention in Iraq to be unpopular – but almost 70 percent of respondents from one 2012 poll say U.S. troops should be deployed abroad if only they can stop genocide.