Hello. You did not enter the text to which this question is related, which makes it impossible for it to be answered accurately. However, I will try to help you in the best possible way.
To support an argument, the author must show information that proves, supports and provides a justification for the argument to be made. In this case, in relation to the sentence shown in the above question, the author must present historical moments where the interaction between colonists and Native Americans was violent, harmful and bad.