The 1940 series, also referred to as 194X and "19" series, is a series of war-themed Shoot'em Ups created by Capcom.
As the series's name imply, they are set during World War II, and as such feature many armaments of the time, from fighters to ships, both as heroes and enemies. Despite this, many anachronistic elements appear, such as weapons and vessels which would only exist after the war was over and even more futuristic weapons such as laser guns.
The first two games in the series are set on the Pacific Theater of World War II, where the U.S. Navy confronted the Imperial Japanese Navy. Due to the controversy over the depiction of the Japanese as enemies on a Japanese game, later titles would be set on fictional battles happening during the war. 19XX: The War Against Destiny would go a step further and be set on an entirely fictional war during modern times.
The 1940 games are vertically-scrolling Shoot'em Ups. While early games used a vertically-oriented screen, later games would use a horizontally-oriented screen. Much like other Shoot'em Ups, the objective is to fight through a stage shooting at enemies then fighting a boss at the end.
Aside of the original 1942 and 19XX, the games ditch the traditional lives system in favor of a fuel/health gauge, one of the series's staples. Whenever the player is hit, a portion of the health is depleted, and the game is over once the gauge is empty. Many weapons are available to the player, which come on a single power-up which cycles with time or whenever it is hit. Another staple of the series are the Side Fighters, smaller ships which follow the player's ship, protecting it's sides and adding to it's firepower.