The flag displays a white cross
in the center of a square red field.
The red shows the color of blood in the battlefield.
The white cross is known as the Swiss Cross:
The white cross means Christian cross symbol
and is the field sign of the Old Swiss Confederacy

buttlefield （戦場） 
Swiss Cross (スイス十字)
the Old Swiss Confederacy  (旧スイス連邦）：