Ahistorical source can be deemed unreliable if the author appears to unfairly favor a particular side or viewpoint. in this case, the source

biased.

Explanation:

Historical research is a very complicated and nuanced kind of research for, in dealing with historical events, it is always difficult to take into consideration the perspective and relevance of all parties involved in a given event. For this reason, it is extremely important to try to corroborate the information gained through historical sources in order to avoid repeating and propagating what is already a biased, and for this reason, unreliable and incomplete, historical view.