Ashton Carter's inaugural trip to Asia as US Secretary of Defense went about as smoothly as he could have hoped.

The logic of visiting Tokyo and Seoul ahead of other capitals in the region is straightforward. Between them, Japan and South Korea host over 80,000 US military personnel and the bulk of forward-deployed assets in the Western Pacific. They also bring more military capability to the table than other US allies in Asia. For these fundamental reasons, the US rebalance remains top-heavily reliant on its Northeast Asian treaty allies, not counting the US build-up on Guam, an American Pacific territory. 

Secretary Carter's introductory trip appears to have avoided unexpected traps or hurdles in the fickle business of alliance management.

His priority in Tokyo was to help steer the latest iteration of the US-Japan Defence Cooperation Guidelines with the Abe Administration into their final phase. Last revised in 1997, the new Guidelines are scheduled for completion by the end of this month. The current internal threat to the alliance comes from the election, in November, of a new Okinawan Governor opposed to the US base relocation plan on the island, exposing an old alliance sore.

Carter's hosts deliberately sidestepped the relocation timetable for this trip, though it will inevitably return as an issue on Carter's watch. That said, shared threat perceptions towards China and North Korea continue to bind the US-Japan alliance. To the extent that doubts permeate through Japan's body politic, these reflect concerns about the reliability of the US security guarantee. 'Gray-zone' tensions with Beijing in the East China Sea since 2010 have raised Tokyo's threshold for reassurance up a few notches.

In Seoul, the substance of this visit was less clear.

With last October's agreement to delay the transfer of wartime operational control over RoK forces, the main thorn in US-RoK alliance management concerns ballistic missile defence (BMD), particularly a US proposal to introduce its Terminal High-Altitude Area Defence system to South Korea. Living close to North Korea's short-range-missiles and Pyongyang's other military threats, the South Koreans are not sure if they need such sophisticated BMD systems that go beyond their own defence requirements. Pressure from China is another factor behind Seoul's lukewarm reception to Washington's THAAD proposal. It was kept off the agenda for Carter's visit.

With BMD cooperation stalled and little else of substance to announce bar ongoing cyber collaboration, Carter instead chose to make a point about alliance solidarity by staging a photo opportunity alongside the South Korean corvette sunk by a North Korean torpedo five years ago. While US military concerns about Pyongyang's development of an 'operational' ICBM capability ran high in the lead-up to Carter's visit to Seoul, the fact remains that North Korea's last flight test of a long-range rocket occurred back in December 2012.

While shared threat perceptions about North Korea still bind the US-RoK 'blood alliance', Seoul's more delicately poised attitude towards China and its political animus towards Japan hold back the alliance's potential to serve as more than a localised defence pact. Trilateral US-Japan-Korea security cooperation has been revived after coaxing from Washington, but Seoul's wavering on BMD exemplifies the limits. 

This gets to a wider dilemma confronting US policy in the region.

Prominent coverage of China's land reclamation activities in the South China Sea during Ashton Carter's visit to Tokyo and Seoul appears to herald a shift of US strategic focus away from Northeast Asian flash points. The US Seventh Fleet Commander suggested in January that Japan could expand air patrols into the South China Sea, and more recently that Japan's embrace of 'collective self defence makes it easier for the Seventh Fleet and JMSDF to exercise and operate across the Indo Asia Pacific'. Secretary Carter reiterated the call for Japan to assist US surveillance missions in the South China Sea, framing the revised Guidelines in terms of their contribution to regional security.

Japan identifies that its own national interests are at stake in the South China Sea, while Japan Self Defence Forces (JSDF) surface ships and aircraft periodically transit through. Japan's submarines, new-generation P-1 maritime patrol aircraft and impending acquisition of Global Hawk surveillance drones will provide added reach. But the Japanese presence has been kept minimal and discreet to avoid 'provoking' China. The US patrol proposal has also encountered push-back from Japan's defence establishment on the practical grounds that the JSDF is already overstretched defending the country's remote islands and maritime approaches.

Given Southeast Asian countries' limited collective patrol capabilities, it seems likely that the US strategic interest in getting its Northeast Asian allies to operate more regularly in the South China Sea will intensify. While India is not a US ally, the same logic could be said to apply in Washington's dealings with its more capable Indo-Pacific naval partner in New Delhi, and also Canberra.

Photo courtesy of Flickr user US Department of Defense.