In 1946, the United Nations declared genocide a crime under international law. In 1948, the UN adopted Resolution 260A(III), the Convention on the Prevention and Punishment of the Crime of Genocide, which obliged the "contracting parties to undertake to prevent and punish... acts committed with intent to destroy, in whole or in part, a national, ethnic, racial, or religious group."
After the horrors of World War II, the world said "never again" to horrific mass killings. But, due to the Cold War tensions, idealistic ideas such as this one were abandoned in favor of realist politics and fighting for self-interests. "Never again" does not mean "we will do everything to stop genocides from happening anywhere in the world." The Western world in particular considers stopping genocides only in countries where they have economic or other interests.
That is why in 1994 the American government did not want to use the term "genocide" to describe the fastest genocide in recorded human history that took over 800,000 lives in Rwanda in only 100 days. This was "the most efficient mass killing since the atomic bombings of Hiroshima and Nagasaki," as Philip Gourevitch puts it. Or, as George W. Bush calls it, Rwanda was a "wholesale slaughter."
Calling the mass slaughter "genocide" would obligate the US and other governments, signatories of the Resolution 260A(III), to intervene and stop it. But the US and other Western countries did nothing because they had no interests in the small, overpopulated, and poor African country. That a whole ethnic group was being exterminated in front of the whole world was not enough.
So the American officials said they knew that the "acts of genocide" were taking place in Rwanda but this was not enough to compel them to intervene. Blocking UN Security Council decisions concerning Rwanda, they even prevented other countries from acting.