The modern era certainly pre-dates the Depression and WWII. The mid-90's to the blackout period (1908-11) can be seen as early modern (Breeders like Keene, trainers like Rowe and horses like Domino, Kingston, Colin, Maskette, Roseben, Commando, Beldame, Sysonby, etc.) and post blackout to WWII (1912-1939) as mid-modern (Col. Bradley and John Madden, Man o'War, Grey Lag, Exterminator, Roamer, Sir Barton, Regret, etc.)
Agree with Doug that the record-keeping element is a good drawing line and the origination of Triangle Publications came right around the time of Domino & Kingston. This whole discussion points out the importance of the preservation project of the UK/Keeneland Library.
http://kdl.kyvl.org/drf/drf-giving.php