“Old” call of duty is based from WWII, which were made in the early 2000s inspired by Medal of Honor while “New” call of duty is the fictionalized takes within the FPS genre (basically the ones you recognize today). Modern Warfare (the 2007 one) is the title that departed from the WWII genre while World at War tackled at both theaters of WWII (the Pacific and European).
Call of Duty: United Offensive is my favorite game of all time.
MW to Black Ops 1 were good games but I still play UO to this day.
We play competitive S&D every Tuesday and Sunday and casual CTF and BAS servers are populated every other day.