- China responded to an exercise involving the US and South Korean militaries with one near the Koreas that it said had never been done before.
- China is asserting its position as Pyongyang's treaty ally amid escalating tensions between the US and North Korea.
- It's in China's national interest to prevent a conflict involving North Korea - and in the Korean War of the 1950s, China fought on Pyongyang's side.
As the US and South Korean militaries worked together on the largest-ever version of their annual air-power exercise on the Korean Peninsula, China did something that suggests it would consider backing up North Korea in the event of war.
On Monday, the same day the exercise started, a spokesman for China's air force said it had staged exercises along "routes and areas it has never flown before" with surveillance aircraft over the Yellow and East seas near the Korean Peninsula, according to the South China Morning Post.
While China needs to exercise its constantly expanding and modernising military, this exercise most likely had another purpose.
"The timing of this high-profile announcement by the PLA is also a warning to Washington and Seoul not to provoke Pyongyang any further," Li Jie, a military expert based in Beijing, told the Post, using the abbreviation for the Chinese People's Liberation Army.
With a record 24 US stealth aircraft in South Korea for the Vigilant Ace exercises, China's move sends a clear message.
The US and South Korea have up to 260 aircraft training to take out important North Korean targets in a realistic simulation of an air war.
While China agrees with the US that Pyongyang's nuclear-weapons progress is dangerous, it fundamentally disagrees on the preservation of the North Korean state.
China, North Korea's main trading partner, could effectively defund Pyongyang and bring about the collapse of the government, as the US has encouraged. But China's national interest lies in keeping North Korea as a buffer state to prevent the US from staging troops on its borders.
In the Korean War of the 1950s, North Koreans backed by China fought US-backed South Koreans in a brutal war that is technically ongoing because it ended in a cease-fire rather than a peace treaty.
China's latest military exercise may serve as a reminder to the US that the two military powers could find themselves on opposite ends once again should conflict break out with North Korea.