Historical texts are, quite naturally, usually written in the past tense. The switch to the present tense can create a sense of immersion in the period described and is sometimes used as a way to 'transport' the reader to that particular period, so that they can imagine themselves in that context. For example, 'It's September 1st 1939 and Nazi Germany have just entered Poland' 'tricks' the reader into imagine the circumstances described are happening 'now'.
