The mission called "No Russian", involves you and a gang of terrorists killing innocent people in an airport. This mission can be skipped by pausing the game and selecting "Skip Mission". It is also warned about as soon as you start up Modern Warfare 2 for the first time, and can be skipped from there. It is also possible to walk through the airport without ever firing your weapon.
For the German and Japanese Version the mission was slightly edited. As soon as you start shooting the civilians the game aborts. The Russian edition of the game completely omitted the mission.