This isn't so much a bug as a whopping great big whole in the design of the Tech tree. I am playing on the default difficulty setting so I am much more advanced than my neighbouring civs. I had machine guns and riflemen, then all of a sudden I hit the Modern Era and I'm left without any ability to create offensive players. I am literally being overrun by catapults and horse archers while I am reseaching flight. The only offensive unit, rilfemen, instantly turn to SAM Infantry mid-production and without notification. SAM Inf. and Artillery are the only units I can now create, none of which can attack land units. Which means I can bombard a city but not take it and I can't clear out the hords of primitive yet invincible units running all over my civ.