The Axis Powers were an alliance of nations during World War II, hostile to the Allied Powers.
The principal three Axis Powers were Germany, Italy, and Japan. They are the main enemy in every Call of Duty game set in World War II, as well as playable factions in multiplayer.
In-game
Call of Duty & Call of Duty: United Offensive
Germany is the only Axis power encountered in Call of Duty and Call of Duty: United Offensive.
Call of Duty 2
Germany is the only Axis Power encountered in Call of Duty 2.
Call of Duty 2: Big Red One
Germany and Italy are featured enemies in Call of Duty 2: Big Red One. Vichy France is the enemy in the mission "Baptism of Fire", though it was not one the Axis Powers.
Call of Duty 3
Germany is the only Axis Power encountered in Call of Duty 3.
Call of Duty: World at War
The Axis Powers featured in Call of Duty: World at War are manifested in the Imperial Japanese Army and Navy and the German Wehrmacht and Waffen-SS. Japanese forces are only seen throughout the American campaign (Pacific Theater) and German forces are only seen throughout the Soviet campaign (Eastern Front).