This is a major American advantage:
The South Korean Navy was originally meant to be a coastal defense force, mainly because the main threat was from North Korea or nearby China. Since the late 1990s it became obvious that all those destroyers and frigates being built in South Korea performed well on the high seas the South Korea Navy was rapidly becoming one of the largest and most modern in East Asia. Together South Korea and Japan pose the greatest naval threat to China. Although the American fleet is much larger the portion of the U.S. Navy stationed in the West Pacific is smaller than the combined South Korean-Japanese force. The Americans have the large carriers, large amphibious ships and nuclear subs but the South Koreans and Japan have most of the subs (all diesel-electric), destroyers and frigates. South Korea already has three large (11,000 ton) Aegis equipped destroyers with three more under construction. Japan also has Aegis equipped destroyers and large destroyers that function as helicopter carriers.
If an enemy starts a war, it will take time to mobilize and deploy American military power to the area of operations, and American allies can help our forward-deployed forces hold the line and contain the enemy assault until then.
Of course, allies with military power are a two-edged sword. America must take care to maintain enough military power to encourage allies to remain loyal against a common enemy rather than defect or just decide to sit a conflict out and take their military power with them.
