I know there were issues about the genocides in the Balkans in the 1990s. But, in that case, he wasn't saying that there weren't massacres taking place -- he was saying that the US was selectively engaging for geopolitical reasons, not humanitarian ones.
He did say that, ignoring the fact that the real reason for this change was the election of Bill Clinton, and he also repeatedly, even in the face of overwhelming evidence to the contrary, denied that Omarska was a concentration camp, he uses mocking language suggesting things like "because there's barbed wire and some people were thin people call it Auschwitz" (as it, in fact, was). He's been called on repeatedly to correct his previous statements and only doubles down.