According to the pilot, an annual inspection was completed on the airplane the day prior to this flight, so he planned to remain in the traffic pattern. During the first landing after touchdown, the pilot reported that the airplane began drifting to the right. He attempted to correct with the left rudder and brake, but once the tailwheel touched down the airplane quickly veered to the right, departed the runway, and ground looped. After the ground loop, the airplane stopped and the pilot shutdown the airplane without further incident. During a post-flight examination, the pilot stated that the right main tire was one half to three quarters underinflated. After further inspection, the pilot identified a small metal object that resembled a finishing nail in the right main tire and within one hour of the accident, the tire was completely flat. The pilot reported that both tires and associated pressures were checked and verified according to the airplane maintenance manual prior to flight. The pilot stated that there were no preaccident mechanical malfunctions or failures with the airplane that would have precluded normal operations, with the exception of the right main tire. The airplane sustained substantial damage to the left side of the fuselage.