In the immediate aftermath of North Korea testing an intercontinental ballistic missile, South Korean President Moon Jae-in ordered talks with the US about adding more Terminal High-Altitude Area Defense (THAAD) missile interceptor systems to South Korea.

While the move may seem like a common-sense defensive measure, it actually indicates that South Korea may be getting ready for a fight with the North.

The THAAD has a perfect record of intercepting shorter-range ballistic missiles in test conditions, but cannot handle the reentry speeds of an ICBM. Furthermore, if a missile fired from North Korea is landing in South Korea, it's not an ICBM.

The THAAD can only block shorter-range missiles, and it does so with a powerful radar that scares the pants off the Chinese. China hates the THAAD because it fears the US could potentially use its powerful radar to negate China's nuclear ICBM force, and thereby erode its nuclear deterrent. China routinely protests THAAD and pressures South Korea economically into giving it up.

So why would Moon take a huge political risk by increasing defenses against missiles that aren't even supposed to target his country?

One possible explanation is that he's moving to defend his country against shorter-range missiles from the North in the event of a strike on Kim Jong Un. Seoul is within range of nuclear and conventional arms from North Korea that hold its 25 million inhabitants hostage.

But if South Korea could block the nuclear strikes, its air force could knock out North Korea's massive artillery installation, and US forces could move in and destroy the country's nuclear infrastructure.