People want to enjoy and feel good about their victory, not learn that their side was responsible for the war or that the war could have been avoided except for the hidden agendas of their own leaders. Historians are also constrained by the unavailability of information. To hide mistakes, corruption, and crimes, governments lock up documents for decades.  Memoirs of participants are not yet written.  Diaries are lost or withheld from fear of retribution.  It is expensive and time consuming to locate witnesses, especially those on the losing side, and to convince them to answer questions.  Any account that challenges the "happy account" requires a great deal of confirmation from official documents, interviews, letters, diaries, and memoirs, and even that won't be enough.  For the history of World War II in Europe, these documents can be spread from New Zealand and Australia across Canada and the US through Great Britain and Europe and into Russia.  A historian on the track of the truth faces long years of strenuous investigation and development of the acumen to judge and assimilate the evidence he uncovers into a truthful picture of what transpired. The truth is always immensely different from the victor's war propaganda.