The pilot reported he was landing from a personal flight in his tailwheel-equipped airplane when it suddenly swerved to the right shortly after the tailwheel contacted the runway surface during the landing roll. The pilot applied left rudder and brake to regain directional control, but the airplane veered off the right side of the runway into a level grass area. The left main landing gear (MLG) spring fractured where the spring entered the fuselage, which resulted in the airplane coming to rest on the lower left fuselage in a left wing down attitude. The left wing, left aileron, and the fuselage sustained substantial damage. Postaccident examination of the left MLG spring revealed a preexisting fatigue crack that initiated from the upper forward edge of the spring adjacent to the fuselage. The fatigue crack likely reduced the overall strength of the landing gear spring which resulted in its separation during the landing roll.