The Main Conventional Threat to the ROK Moved to the PRC

South Korea will add to their naval heft with 3 more Aegis destroyers and 3 submarines. Because China's air and naval threat to South Korean sea lines of communication increases while North Korea's ground threat to Seoul decreases.

This decision is part of a long trend in South Korean military development:

South Korea on April 30 approved the construction of a second batch of three KDX-III Sejong the Great-class multi-purpose destroyers and three more indigenously designed KSS-III (Jangbogo–III-class) diesel-electric attack submarines (SSKs).

The Defense Project Promotion Committee of South Korea’s Defense Acquisition Program Administration (DAPA) approved the $6.3 billion procurement last month. The new destroyers and submarines are expected to join the Republic of Korean Navy (ROKN) in the late 2020s.

The three new KDX-III destroyers will be equipped with the Aegis combat system and an upgraded missile launch system that will enable the warships to conduct ballistic missile defense. The surface combatants will feature “improved naval and anti-submarine warfare capabilities,” according to DAPA.

Missile defense is of course useful against North Korea, too.

But the South Korean naval build up has been going on for a while as North Korea's ground threat withers.