Here numbers, unfortunately, don't lie. The US spent US$425 million (S$577 million) over five years on the Maritime Security Initiative designed to boost the maritime capabilities of South-east Asian partners. In comparison, it spends more than US$10 billion a year alone on Afghanistan. In other words, it is a matter of degree, not principle.

None of this is to suggest that the US should downgrade the South China Sea in its order of priorities.

On the contrary, I believe there are sufficient reasons for the US to still take seriously its role in the South China Sea as a non-claimant but interested party. Quite apart from the potential oil and natural gas deposits in the South China Sea, which obviously would be of interest not just to the US, at stake is the principle of freedom of navigation in a body of water through which a significant amount of commercial shipping passes to drive the economies of important trading partners. The American political and military leadership recognises this with its common refrain: "The US will fly, sail, and operate wherever international law allows."

Second, it has always been a strategic objective of the US to prevent domination of any given region by a single power. That this logic should inform American interest in the South China Sea should be self-evident.

Third, although President Trump has downplayed the merits of international trade, over time it will be hard for him to ignore the economic dynamism and prosperity of the East Asian region, or to deny that partaking of this dynamism and prosperity will be very much in the vital interest of "America First".

PARAMETERS FOR A U.S. STRATEGY

So, with all this at stake, what should a comprehensive American South China Sea strategy look like? Here are some thoughts on possible parameters.

• Founded on international law

First, the administration should work out an approach that is predicated on international law. International law provides the vantage from which Fonops and, in the event of any declaration of an Adiz (Air Defence Identification Zone) by any claimant state, overflight operations can and should be conducted. Statements underscoring the importance of unimpeded access to global commons must be repeated, and, when necessary, backed up by action.

At the same time, Washington should be mindful of its own chequered history with international law. The US has yet to ratify Unclos - the United Nations Convention on the Law of the Sea - that it maintains forms the legal bedrock governing competing South China Sea claims, is not party to the Rome Statute of the International Criminal Court despite playing an instrumental role in crafting it, and has never been willing to submit itself to the International Court of Justice even though it has appeared before it on several occasions.

These are of course not deal breakers, and at least on Unclos, the US has by and large adhered to its principles. But they do cast a pale shadow over America's moral leadership and authority to comment on the sanctity of the rules-based international order. Suffice to say, China seldom passes up an opportunity to remind the US of this.

• Deterrence and non-military measures

If international law forms the first prong of a comprehensive strategy, deterrence should be the second.

Following the July 12, 2016 Arbitral Tribunal award on the South China Sea, some have suggested that because the legal status of features in the Spratly Islands chain of the South China Sea has been clarified, there is less need for Fonops.

This view implies the award has been accepted by all concerned. This, as we know, is not the case.

Furthermore, this view also disregards the deterrence value of Fonops, which is a necessary, if imperfect, tool to dissuade claimant states from prosecuting their claims via military means. To that end, the US Navy should continue to maintain a robust yet carefully calibrated presence in the region.

But deterrence should not be limited to Fonops or, indeed, to military measures alone. Non-military measures are equally pertinent. For this reason, it is imperative that the naval presence is synchronised with economic and diplomatic measures, lest the impression is created that the US is just as culpable for the "militarisation" of the South China Sea.

The economic toolkit, in particular, has been grossly under-explored, let alone utilised. It is striking, for instance, how the Obama administration's Lower Mekong Initiative, designed to enhance the economic potential of the Lower Mekong sub-region, started with a bang but has since been reduced to a whimper.

The point is that not only is economic policy useful as a form of deterrence (think: sanctions), if expressed as investments commensurate with the size of the American economy - for all China's advances, the US remains one of the two largest economies in the world and dominant in technology - it will also demonstrate the reliability of the US as an important economic partner for South-east Asia. In turn, this would afford the US added leverage in multilateral discussions on regional security issues.

Ultimately, whether we are talking about military or non-military deterrence, the US must be clear about the costs it is prepared to impose in response to specific actions by any party in the South China Sea, and demonstrate the necessary resolve to dispel any attendant ambiguity. Nothing less than US credibility is at stake.

• Incentives

Third, incentivisation. Deterrence can co-exist with cooperation, so while it is necessary to impose costs on adventurism in the South China Sea in order for deterrence to be effective, it is equally necessary that clear incentives for restraint are factored in.

A useful way to think about this is to consider coupling issues like the South China Sea with wider efforts currently under way to re-examine the Sino-US bilateral economic and trade relationship. This coupling would be done in a strategic manner, not for purposes of using the South China Sea as leverage or a bargaining chip, as some regional parties fear, but rather to provide China incentives to preserve the status quo in the South China Sea without compromising American interests or touching upon the sensitivities of sovereignty claims (where the US has no dog in the fight).

Realistically, however, this would require President Trump to adopt a more sophisticated, far-sighted view on trade and (domestic) economic policy than he is currently inclined.

• Diplomatic engagement

Which brings me to the fourth point - hastening of diplomatic engagement. While it is well and good that elected officials from the Trump Cabinet publicly iterate reassuring statements, exchanges at the level of senior officials with South-east Asian counterparts still need to be enhanced.

Because the devil, as always, is in the detail, diplomatic engagement will reinforce reassurance messages with important corresponding discussions on practical policy measures. Unfortunately, there is at present a discernible slowdown in interactions at the senior officials' level between the US and its regional partners, given the considerable number of offices that remain unoccupied across the Trump administration.

• Keep Asean central

Finally, the US should be mindful that any approach to the South China Sea should not get ahead of Asean. What this means is that Washington's crafting of any policy on the South China Sea would be served by some measure of consultation with Asean states. By engaging South-east Asian counterparts in this manner, Washington would also be sending the right signals that as a non-claimant external major power interested and involved in regional security issues, it is prepared to listen to and consider South-east Asian perspectives.

• Joseph Chinyong Liow is dean and professor of comparative and international politics at the S. Rajaratnam School of International Studies, Nanyang Technological University.
  He is the author of Ambivalent Engagement: The United States And Regional Security In South-east Asia After The Cold War (Brookings Institution Press, 2017).