There has arguably not been a US presidential election with so much at stake for the rest of the world since the second world war.

One candidate stands largely for continuity, signalling she would do slightly more than her predecessor to shore up the existing liberal world order.

Her opponent is vowing to turn that order on its head, jettisoning alliances and free trade agreements, while seeking to cut deals with the world’s authoritarian powers.



If Donald Trump is elected on Tuesday, it is likely to trigger a foreign policy crisis inside Washington, as the diplomatic and security establishments scramble to discover whether he meant what he said on the campaign trail. Those who believe that the election talk was all bluster and that Trump will tack back to a more traditional Republican foreign policy once elected are likely to be disappointed, according to Thomas Wright, an analyst at the Brookings Institution in Washington.

“My view is he is a more ideological candidate than we often appreciate and while there is a lot of bluster and a lot of ignorance on a wide number of issues, there are certain things that he has been pretty consistent on,” Wright said. “I think he is opposed to US alliances ... and I think he would try to withdraw the US from security commitments around the world. He is anti-trade and he would veer towards a more mercantilist system, and he is pro-Russian – he is pro-authoritarian.”

Some believe that Trump would be confronted with a series of resignations by diplomats and generals if he tried to turn these campaign platforms into administration policy.

If, on the other hand, Hillary Clinton wins the electoral college vote, she will face the opposite dilemma: how to distinguish her foreign policy from Barack Obama’s. She has expressed an inclination toward a tougher stance in Syria and in Europe, for example, but the wiggle-room for a change of policy has shrunk with every passing week as Vladimir Putin has sought to create new facts on the ground in the last stages of the Obama era. More weeks would go by while a new Clinton administration dives into an inevitable policy review.

However, events around the globe may not wait that long. Whoever sits down at the desk in the Oval Office on 20 January next year may be forced to take some of the toughest decisions any US president has taken, much sooner than he or she would want.

North Korea

US power and influence is being challenged around the world at an accelerating rate, and nowhere more so than on the Korean peninsula. The North Korean leader, Kim Jong-un, has sped up tests of nuclear weapons and the missiles to deliver them. Expert opinion varies on whether his technicians have succeeded in making a nuclear warhead small enough to put atop a missile, and there is a general consensus that if they have not done so yet, they will soon. It is also widely anticipated that in the next few years, North Korea will perfect an intercontinental ballistic missile capable of reaching the US west coast. Former and current US administrations have taken the strategic decision that Pyongyang should be prevented from reaching that point, by force if necessary. But the ultimate decision will most likely be taken by the next president.



Clinton’s foreign policy advisers have signalled that she would focus on the looming Korean crisis early in her term in office, reassuring Japan and South Korea that the US will not abandon them, and seeking to put pressure on China to enforce UN-mandated trade sanctions more effectively. That may well not succeed, leaving a Clinton administration with the choice of agreeing to a resumption of dialogue with Pyongyang and accepting its nuclear weapons status, as long as it restricts missile development. If that were possible, it would alarm US allies who are already within North Korean missile range.



American allies are likely to reach that point of alarm far more quickly if Trump reaches the Oval Office. He has questioned the value of US alliances in the region, and he has floated the possibility that countries like Japan or South Korea should develop their own nuclear weapons rather than rely on the US strategic umbrella. If he sticks to that policy in office, there could be a sudden nuclear arms race in north-east Asia and possibly beyond. Ultimately, a Trump administration would have to face the direct threat of a North Korean long-range missile. He has said he would put more pressure on China to get tough with its neighbour, and he has said he is willing to meet Kim in the US, on the grounds “there’s a 10% or a 20% chance that I can talk him out of those damn nukes”.



Failing that, he has signalled he is quite willing to contemplate using the US nuclear arsenal. If the weapons cannot be used, he asked: “Then why are we making them? Why do we make them?”

China

Within his or her first year in office, a new US president would also face a direct challenge to US power in the western Pacific. The Chinese programme of laying claim to reefs and rocks in the South China Sea and turning them into naval and air bases gives Beijing potential control over some the busiest shipping lanes in the world. US influence is under further threat by the rise of Rodrigo Duterte in the Philippines, who has threatened to eject US troops, casting doubt on his predecessor’s agreement to allow new permanent American presence.

Clinton’s likely policy will be to continue Obama’s faltering “pivot to Asia”, and to prioritise restoring the faith of US allies in the region that Washington will help them resist Chinese attempts to dominate the South China Sea. It is a policy that is held hostage to some extent by Duterte’s ultimate intentions, and it could lead to a rapid escalation of tension in the region.

Trump has pointed to the Chinese reef-building programme as a reflection of US weakness but has not said what he would do about it. He has focused more on the threat posed to the US by its trade relations with China. In the transactional model of foreign relations Trump favours, he could agree to turn a blind eye to creeping Chinese takeover in the South China Sea in exchange for a bilateral trade deal with Beijing on better terms.

Syria

A new US president will arrive in office at a time of significant military advances against Islamic State in Syria and neighbouring Iraq, but diminishing options when it comes to helping shape the opposition battle against the Assad regime and its Russian and Iranian backers. It is possible that the rebel stand in Aleppo will have fallen by then, giving the regime the upper hand and postponing yet again any hopes of a political transition.

In the presidential debates, Clinton talked of establishing a “no-fly zone” or a “safe zone” inside Syria. However, it is hard to see how that would be done without risking a direct clash with Russia, with all the risks that entails. The generals at the Pentagon, who have long argued against the feasibility of establishing such a zone, would work hard to block such a scheme. A Clinton White House is also likely to explore ways of increasing the flow of arms to moderate opposition groups.



Trump has indicated that he would seek to work with Assad and Putin in a combined fight against Isis, and has not voiced criticism of the bombardment of rebel-held areas such as eastern Aleppo. That policy would also have heavy costs. The Syrian opposition and the Gulf states would see it as a betrayal, and the new administration would have to deal with the reality that neither the regime nor Russia has much immediate interest in fighting Isis.

Russia and Ukraine

A Clinton administration is expected to take a tougher line with Moscow than the Obama White House, all the more so because of the substantial evidence of the Kremlin’s efforts to try to intervene in the US presidential election in her opponent’s favour. Clinton could well seek to take a leadership role in negotiations with Moscow over Ukraine and the stalled Minsk peace process, which have hitherto been left to Germany and France. She could also opt to send lethal aid to Ukraine as a way of increasing US leverage.



Trump is likely to take the opposite approach. He avoided criticism of Russia for its actions in Ukraine, hinted he might accept the annexation of Crimea, and ignored US intelligence findings that Moscow was behind the hacking of Democratic party’s email. A Trump administration is unlikely to contest Russian enforcement of its influence in eastern Ukraine.

Europe and Nato

Clinton aides have signalled consistently that one of her priorities would be to show US willingness to shore up EU and Nato cohesion, and will attend summits of both organisations in February.



Trump has suggested, by contrast, that Nato is obsolete and questioned whether its security commitments in Europe are worth what the US is currently spending on them. He said he would check whether US allies “fulfilled their obligation to us” before coming to their defence, calling into question the purpose of the defence pact. Later in the campaign, he changed tack, saying he would seek to strengthen the alliance, but a win for Trump on Tuesday would nonetheless deepen anxiety in eastern European countries, such as the Baltic states, that a US-led Nato would come to their defence in the face of Russian encroachment.

Trade

The two major free trade projects of the Obama administration, the Transatlantic Trade and Investment Partnership with Europe (TTIP) and the Trans-Pacific Partnership (TPP) with the countries on the Pacific rim, will probably still be under negotiation when the new president comes into office, giving him or her the option of killing or completing them.



Clinton first supported the TPP and then criticised it in the face of the primary challenge from Bernie Sanders. Her reservations may prolong the negotiations, but she is ultimately expected to pursue and seek completion of the ambitious multilateral trade deals.

Trump built his campaign on opposition to all such deals, which he has characterised as inherently unfavourable to the US. He has promised to seek bilateral trade deals on better terms and to punish other countries deemed to be trading unfairly with sanctions, ignoring the threat of retaliation.

Climate change

Trump has promised to withdraw the US from the Paris climate accord, which comes into force on Friday, and to “stop all payments of US tax dollars to UN global warming programs”. The Republican presidential nominee has said he’s “not a believer in climate change”, describing it as a Chinese-created hoax, and has castigated efforts to reduce greenhouse gases as a risk to American jobs.



The prospect of Trump in the Oval Office poses a unique challenge to the Paris deal, which was struck last year and ratified in speedy fashion in October. A total of 94 nations have fully signed up to a commitment to keep global temperatures below a 2C increase on pre-industrial times, in the hope of staving off catastrophic sea level rises, heatwaves and extreme weather.

The emissions cuts pledged by countries currently aren’t enough to avoid climate calamity, meaning that recalcitrance from the US, the world’s second-largest emitter and the “global leader” in fighting climate change, according to Barack Obama, would risk ossifying a process that needs a further hefty shove.

American involvement in the Paris deal – which has the support of nearly 200 other countries – is based upon the presidential scrawl of Obama. The accord wasn’t presented to Congress as a formal treaty, meaning that Trump could undo it himself.

Extricating the US from the deal, along with its financial support of countries particularly vulnerable to climate change, would take some time, however. Now the agreement is in force, any signatory nation faces a three-year delay before it could opt out, with a further one-year wait as the process plays out.

“That means,” said Robert Stavins, an expert at environmental law at Harvard University, “it would be impossible for a President Trump to remove the United States during what would be his first term.”

But the uncertainty would undoubtably spook countries that rushed to embrace the Paris deal after the US and China, the world’s two greenhouse gas superpowers, jointly announced they were committed to the process last year. Some countries, aware of the dangers of climate change, may redouble their efforts to slash emissions but others may use it as a convenient excuse to slack off.

Trump’s antipathy to climate action is underscored by fierce support for fossil fuels, telling coal miners that he will “put you back to work” and that “gas will be a beautiful thing” in his administration. It’s also likely that he would scrap Obama’s Clean Power Plan, the centerpiece policy to curb emissions from power plants, and allow projects such as the controversial Dakota Access pipeline, in which he is indirectly invested. US emissions, which have begun to level off in recent years, could start climbing again.