Exclusive: Flight mission near North Korean border exhibits US motto there: 'Fight tonight' ABC News flew with the U.S. military to the North Korean border.

 -- The motto of the more than 28,000 U.S. forces stationed in South Korea -- "fight tonight" -- emphasizes their readiness to mount a defense against a North Korean threat at any time.

ABC News' Chief Global Affairs Correspondent Martha Raddatz saw how seriously U.S. forces take that saying while accompanying service members on a F-16 flight along the North Korean border this week.

Raddatz participated in a training operation with the 36th Fighter Squadron, a part of the 51st Fighter Wing, which simulated a North Korean invasion into South Korea.

The F-16 crew practiced giving backup to ground troops in need of close air support by conducting fake strikes on air defenses and tanks from a restricted air space known as P-518, which is just 10 miles from the North Korean border and as close as U.S. pilots can get to the regime's territory.

While no bombs were dropped during the simulation, a real Army unit was on the ground calling in the fake airstrikes.

The purpose of the restricted air space is to control aviation operations and prevent “inadvertent overflight of non-friendly borders,” according to U.S. Forces Korea.

"It must be strange, we're not really in a war zone, but it could happen at any time," Raddatz said to the pilot, call sign "True" Daniels, during the flight mission.

"I'll tell you, it definitely gives you a real purpose for waking up in the morning," Daniels responded, adding, "And that's our mission here, is to be ready to fight at a moment's notice."

The 36th Fighter Squadron flies approximately 170 sorties, or flight missions, from Osan Air Base in South Korea every week.