If you thought 2018 was a bumpy ride for US defence and security policy, get ready for an even more risky and dangerous 2019.

Donald Trump is his own ultimate wildcard. The US President is the least predictable element of his own administration.

Take Trump’s Boxing Day visit to US troops at al-Asad air base in Iraq’s Anbar province, which has been a critical part of the fight against so-called Islamic State. Trump gave a trademark chaotic media conference and spent about half an hour talking with soldiers.

In that time he said he had approved so much new equipment for the military that “your eyes are popping”, and that he had fought against the establishment to deliver a 10 per cent pay rise to US servicemen and women. Neither claim is ­remotely true.

Trump then said the US would not leave Iraq, that al-Asad base could be used “if we wanted to do something in Syria”, but it was OK for American troops to leave that country because Turkish President Recep Tayyip Erdogan “wants to knock out ISIS”.

Bizarrely, he added: “And Saudi Arabia just came out and said they’re going to pay for some economic development, which is great. That means we don’t have to pay.”

Trump then left Iraq without the common courtesy of meeting Prime Minister Adel Abdul-Mahdi, whose office said a meeting had been scrapped because of “differences in points of view” over logistics. Read that as Trump refusing to travel 160km to Baghdad.

Iraq’s parliament is now calling for US troops to be withdrawn because of the President’s “arrogance and violation of Iraqi sovereignty”.

Rounding off the trip, Trump posted a video on Twitter revealing the identities of covert US Navy SEAL special forces at the base.

Whatever his intentions in making the visit, Trump’s indiscipline in front of cameras, combined with a complete incapacity to understand the basics of Middle East security, is a serious risk factor for global stability.

Trump’s “America first” policy will not be advanced by leaving Syria to the control of the Iranians and Russians. From Ankara, Erdogan’s focus is not Islamic State but rather to prevent Syria’s Kurds — a key partner of the US military — from strengthening their position along the border with Turkey.

Regardless of what Trump may have thought he heard on cable television, the Saudis have made no new promises to fund economic development in Syria, and nor will they for as long as their Iranian enemies are propping up Bashar al-Assad in Damascus.

As always, Trump’s key audience is his domestic voter base. He is sharpening his message that “we’re not the suckers of the world”.

He told the troops: “While American might can defeat terrorist armies on the battlefield, each nation of the world must decide for itself what kind of future it wants to build for its people, and what kind of sacrifices they are willing to make for their children.”

That chilling message jettisons the US approach to global security since 1941. With Defence Secretary Jim Mattis’s departure, Trump has rid himself of the last obstacle to redesigning a US strategy whereby allies are dispensable, dictators are admirable and deals can be cut with competitors if it saves money.

Trump told journalists in Anbar: “As you know, we’ve ­already built a lot of wall.”

That’s debatable as far as the Mexican border is concerned, but there couldn’t be a more accurate statement of the barriers he is erecting around America’s ability to confront its enemies and to work with its friends.

Where will this development take US strategy in 2019? Aside from the Middle East, where he is really continuing the Obama policy of short-term disengagement at the risk of long-term strategic costs, I see four major challenges for Trump: China; North Korea; a collection of second-tier auto­cracies behaving badly; and finally the concerned allies.

China has plenty of domestic problems but it’s the closest thing the US has to a near-peer competitor, as an increasingly strong military and a population fuelled by Communist Party propaganda look to settle old regional scores and cement a global leading role for Beijing. I hear many experienced officials in Canberra denying that this toxic combination could lead to conflict, but that thinking is an ­artefact of a region that hasn’t seen conventional war for almost 50 years.

In the East and South China seas and now in the South Pacific, in space, cyberspace and in international institutions, China is ­challenging the rules-based international order.

At its core, this order is sustained by US military power.

Outside of the Oval Office there is bipartisan support in congress and throughout the national security community for the idea that stronger US pushback is needed to stop China’s intellectual property theft through cyber and human espionage.

Vice-President Mike Pence has led this charge through some well-crafted speeches. The key question is: what is Trump’s position?

Trump has at least twice indicated he could undermine this approach in the interests of cutting a trade deal with China. The first time was in allowing banned Chinese telco ZTE to continue trading in the US, and the second was the suggestion he might prevent legal proceedings against a senior Huawei executive arrested in Canada.

Trump’s on-off bromance with Chinese President Xi Jinping puzzles Beijing, which looks for clear policy directions.

The risk for the rest of Asia is that the interests of countries such as Japan, South Korea and Australia might equally be disregarded by Trump in the interests of a trade deal with Xi.

Problem No 2 is North Korea. Trump says he looks forward to another meeting with North Korean dictator Kim Jong-un. But with the exception of the President and Secretary of State Mike Pompeo, no one believes the North intends to verifiably and comprehensively get rid of its nuclear weapons and missiles.

We are therefore potentially facing a repeat of the crisis early this year, when the US started seriously looking at options to pre-emptively strike the North’s nuclear and missile facilities. Trump’s summitry with Kim brought the supreme leader international credibility, weakened sanctions, gave China more leverage, and allowed Pyongyang more time to perfect its weapons by all means short of testing them.

The illusion of a US-North Korea process to discuss disarmament could quickly give way to another round of nuclear brinkmanship. That’s been the North Korean playbook for 30 years, but never before with a credible nuclear arsenal able to threaten the US mainland.

Challenge three: the badly behaving autocracies, most prominently Russia, but with Iran, Saudi Arabia and Turkey also in the mix. Trump is troublingly tolerant of autocrats in ways that suggest he has little time for a values-based approach to foreign policy of the type that Australia champions.

All White House administrations balance pragmatism and principle, but how is Seoul meant to react when Trump casually offers to the North a pause in US and South Korean military training?

Will Syrian and Iraqi Kurds be willing to work with US military forces when the Commander-in-Chief abruptly ends the deployment? What about Canadians, French, Germans and others, who see their alliance commitments dismissed as free riding?

The unhinging of US values as a cornerstone of US foreign policy deeply troubles the allies and gives succour to the autocrats. Russian President Vladimir Putin was the only world leader who welcomed Trump’s Syria announcement.

The thinking in capitals as diverse as Riyadh and Ankara must surely be that you can get away with anything as long as you are buying US goods.

So to the allies. Even though Australia will be distracted by an election in the first half of next year, Trump’s injunction that every country must decide “what kind of sacrifices they are willing to make for their children” is as clear a signal as we will get that we need to think harder about our self-­reliant defence capabilities.

Trump is actually quite right: too many countries, Australia included, have been free-riding on America’s security investment.

Whatever the shape of our government after the election, an ­urgent priority must be to rethink how much we spend on defence and how we balance long-term investment for the 2030s and beyond with the need for a force able to conduct demanding military operations today.

Lifting defence spending from 2 per cent of gross national product to 2.5 per cent or even 3 per cent makes a stronger case in Washington that the alliance effectively pays for itself.

We need to make the case for the alliance as compelling to US policymakers as it is to Canberra officials. And we need to be clear-eyed in asking what we are prepared to do for our own security in a world where Trump’s “America first” may cement itself as the norm, rather than the aberration we should all hope it is.