Video report by Senior International Correspondent John Irvine
ITV News has witnessed first-hand the intensifying preparations of South Korea's military as it gears up to respond to an attack from North Korea.
In the otherwise secretive state, South Korea's neighbour has made little attempt to keep its nuclear and ballistic missile ambitions under wraps; only last week, officials in Seoul claimed a ballistic missile had been fired into the waters off the east coast of South Korea.
Less than 50 miles south of the border between the two nations, jets are circulating the skies as pilots hone their combat skills just in case.
A shaky peace has existed between the neighbouring countries for 64 years, but over the last week the tempo of sorties has quickened in tandem with simmering tensions.
Last Wednesday's alleged ballistic missile test came just days after US President Donald Trump said the US was ready to tackle North Korea alone in order to eliminate any nuclear threat the country posed.
On Sunday it emerged that the US had deployed warships to the Korean peninsular as concerns grow over what North Korean leader Kim Jong-un will do next.
South Korea, too, is poised.
The slogan at Osan airbase is 'ready to fight tonight' and it is here that pilots are honing their combat skills just in case.
The South Korea capital Seoul is home to 10 million people who are well within range of the North Korean artillery batteries, which planners believe would open fire on the city in response to a preemptive strike.
One man who used to work for Kim Jong-un's regime before defecting told ITV News nothing will persuade him to abandon his nuclear ambitions.
"History has proved that the North Korean state will never give up its nuclear capability and their final goal is the unification of Korea on their own terms," he said.
Americans will be hoping last week's attack on a Syrian airbase in response to the chemical attack on civilians will give Kim Jong-un pause for thought.
Whether it will or not is another matter altogether.