What happened is a Fact .  Why it happened is subject to Opinion .
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If the person who initiated an event says why they did it, we may teach that as Fact .
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It is important that what happened be taught as Fact . We learn valuable lessons from historical Facts .
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It is important that Opinion never be taught as Fact .  Opinion can shape Facts to suit its own purposes.  
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When a person combines Fact and Opinion in writing about or teaching History they are teaching you not only what happened but what to Think about what happened.
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This is dishonest, manipulative and damaging to the student and hence, Immoral .