Your orders from the HQ were to kill as many Germans as possible to allow them to relocate away from battlefield, so you have killed 100 Germans so far to help them move away. United States high-command will reward you with this ribbon as thanks for helping to stop the Nazi attack and start training soldiers a little bit better to help you kill even more Germans.
Getting this medal is very easy and it provides a nice bonus of 3 points added to the rifle skill of any allied unit deployed. You usually receive this at about wave 30, depending if you receive blitz waves or Detailed Orders and it gets lot better with Medal of Honor. This ribbon should not be confused with the Close Combat Ribbon.