Jaakkola is known as a historian who primarily researched medieval history and sought to put forth that Finland existed as an entity already during that period.

He emphasized Finnish position in the Western sphere of influence and concentrated on the history of his home region, Satakunta.

[1] The central theme of the series was the medieval political history of Finland, during which, as Jaakkola argued, there was a period of ancient independence during which the country guarded its own interests.

[1] After the Winter War, Jaakkola wrote the concise presentation of Finnish history, Suomen historian ääriviivat, which was meant for readers abroad.

During the Continuation War, President Risto Ryti commissioned Jaakkola to write Die Ostfrage Finnlands (1941), to argue for Finnish territorial expansion eastwards to the Kola Peninsula and East Karelia.

[5][6] Despite the difficult style and academic nature of Jalmari Jaakkola's works, his influence on contemporary beliefs is considered to have been extensive.