UNITED NATIONS — The word genocide sends shivers down the spines of United Nations diplomats, resurrecting the memory of the world body’s failure to stem massacres in Rwanda nearly 20 years ago and, before that, in Bosnia.

Lately, in describing the carnage in the Central African Republic, diplomats have been swallowing that word even as it seems to be on the tip of their tongues. They are choosing other words. “Mass violence,” was Secretary General Ban Ki-moon’s choice.

“History has taught us the worst may happen,” is how the French ambassador, Gérard Araud, referred to it.

“There have been atrocities committed already,” said Samantha Power, the American ambassador.

What vexes the world body now is how to prevent another catastrophe in the battered Central African Republic — and in turn how to shield itself from future reproach. That reckoning comes as Western militaries are tied up on other battlefields, and crises elsewhere have made new demands on United Nations peacekeepers and the rich countries that finance them.