Answer:
(3) anti-Semitism and ethnic hatred remain powerful forces in the modern world
Explanation:
(1) world opinion is effective in stopping genocide: the world opinions didn't stop the genocides. They still occurred.
(2)countries generally use reason and negotiation in dealing with important ethnic issues: Nope. Killing thousands and millions of people is not using reason and negotiation. Its using violence and propaganda. This is definitely incorrect.
(3)anti-Semitism and ethnic hatred remain powerful forces in the modern world: Its true. The Holocaust occurred because of anti-Semitism and it wasn't too long ago*. The "ethnic cleansing" in Bosnia was a result of ethnic hatred. And it too wasn't that long ago*.
(4) military leaders cannot be held responsible for acts committed during wartime: Definitely incorrect. They really can absolutely be held accountable for acts committed during wartime. And they were.
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* "wasn't that long ago" explains, and indicates that it occurred in the modern world.