Multiple sources, first hand being the best. With enough, you can even see the context of why a source was written. Julius Ceasar led an army into Gaul, defeated multiple tribes, and spent years consolidating his conquest. We know this because he wrote to Rome reporting what he did. We know he didn’t make it up, because Roman legions kept very good records, and many people on that expedition wrote about it later. We even know why he wrote the way he did (propaganda), because of of all the other people from many perspectives who wrote about it as well.
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