OSAN AIR BASE, Republic of Korea -- The night erupts with the sudden flare of muzzle flashes and the thundering crack of gunfire as opposing forces swiftly descend upon a defensive fighting position full of unsuspecting Airmen.

It's the 51st Security Forces Squadron versus -- the 51st Security Forces Squadron in a simulated, week-long battle for control of the base during the peninsula-wide exercise, Vigilant Ace 16.

SFS trainers utilize newly arrived defenders to fill the role of OPFOR during exercises.

"We use the new guys because it's more realistic," said Tech. Sgt. Jacob Thomas, 51st SFS NCO in charge of training and nighttime OPFOR director. "They just got here and don't really know the layout of the base and while they know some tactics, they don't know the playbook here yet."

Thomas explained that the trainers use this opportunity to observe and evaluate the existing skill level of their new defenders prior to training them to defend the base.

"Getting a perspective from the other side helps," said Airman 1st Class John Williams, 51st SFS member, who was playing OPFOR. "We get to understand the weaknesses that we (Security Forces) would have if an enemy was going to attack us."

Searching out these weaknesses and testing the effectiveness of combat readiness, weapons handling and DFP procedure training is, the main reason for the OPFOR attacks.

OPFOR members stay busy during the exercise by ambushing unsuspecting DFPs and occasionally stick around to fight to the "death" with reinforcements.

"Every time you hear 'ground attack' that's us, going at it," said Thomas.

With OPFOR controllers throwing colored smoke grenades and setting off various ground burst simulators, these attacks turn into a disorienting assault of sight and sound. The effect creates a simulation that is as close to a real battle as possible, though with the added fun for participating OPFOR of being resurrected before moving on to the next engagement.

"This is the hands on stuff that I enjoy doing," said Williams. "This is why I chose security forces."