After COD milked future dry they've turned to knob the past events, so most probably WW2, as the only big difference is a few more curvy tanks and less silly hats. Overall I reckon it might be modern, maybe harping to the Afghanistan and Iraq wars.
Starting to be like COD now where all the games start looking the same. I think this is the first BF where I'll give it a miss (started doing that with COD years ago).
Not to mention BF1 doesn't run as well as it should on my PC, which seems to be a common issue.