Call of Duty: Vanguard

In the mission "Numa Numa Trail" you are rescued by the 93rd Infantry Division. One of the troops in the 93rd is named Lewis Howard. Lewis's brother, Major Howard, is in Call of Duty: WW2. Lewis says a line of dialogue confirming this.

Throughout the campaign there are multiple weapons that appear outside their historical timeline. Such as an MG-42 appearing in 1941 Tobruk, along with a Gustloff Volkssturmgewehr that only appeared in 1945, just before the war ended in Europe. A German Becker revolving shotgun, called an Einhorn Revolving shotgun in game, in the hands of Japanese soldiers. Japanese 6.5mm machine guns on a German train in Europe. Owen submachine guns in North Africa when they were only in the Pacific. Plus many, many others.

Followed by Call of Duty: Modern Warfare II (2022)