The Second World War saw the first steps of the helicopter as a weapon of war. The following link provides a report on this topic and a lot of photos of different models operated by the countries involved in the conflict:
Glad the posted link includes the Kayaba Ka-1 and Ka-2, because these were the first combat rotary-wing aircraft, although they were autogyros rather than true helicopters because they were based on imported Kellett autogyros.