World War 2 was fought between 1939 and 1945. Ok. No one can deny that - there may be official declarations in government offices to testify that. But why was it triggered? Which side was the good side? Who benefitted and who suffered? - All these questions are answered subjectively. Plus, the numbers themselves are of no good if we cannot derive any conclusion from them, learning from it (which probably is the purpose of studying History) - which will again be a subjective matter. Even the quantifiable facts that we say are objective (like the period of the war, the number of casualties, nations involved, etc) are recorded by humans. And hence authenticity of these records gets debatable for there is always be a possibility of motivated recording.
Let's take an example. Ashoka has written in stone (literally) that in his battle with Kalinga a hundred thousand died. But one can always pose the question - was he swelling that figure up to dramatize the event which was turning point of his life? Historians reach a certain conclusion based on multiple pieces of evidence. Sometimes, due to lack of any other supportive evidence, historians are put into fix whether to believe the inscribed records, such as Ashokan Edicts. Kaling war, mentioned in Ashoka's edicts is not mentioned in any of the Buddhist records, which are a major source of information about the Emperor!
The point is, any event, when recorded, is recorded by humans. And hence, even if the facts in a record may be objective in nature, there is always a possibility that the record itself is corrupt or morphed.
But that does not mean make History always unreliable or uncertain. Historians are on a constant lookout for anything related to past events. And when many independent pieces of evidence form one concrete idea about the past, only then they proclaim that idea as fact. Taking the examples we discussed above, if historians, in days millennia from now, if find all the declarations of war by participants of WW2, dating the same year, they will be able to conclude that indeed the war started in 1939. But as we have just one evidence (which, although, is a very strong one) about Kaling war, there is room for doubt about it.
But yes, as it deals primarily with humans, and is also based on accounts recorded by humans, History is largely subjective.
History is definitely subjective because there are many aspects and facets of an event, person, etc., and not all of them can be considered. A person looking at those events selectively highlights the aspects that he or she considers worthy and ignores the rest.