This is where we go off script. Never in the past century have the allies of the United States feared more or known less about a new arrival in the White House. Never in the modern era has a more impulsive, inexperienced and volatile leader assumed the world’s most powerful office. And never in the postwar era have the fundamental tenets of American foreign policy – and, by extension, the world order – looked shakier.

Every US president comes to power facing a complex, messy and uncertain world. The White House always finds itself at the mercy of external shocks. But every president since Harry Truman has also operated within a broadly similar frame of reference in its dealings with the rest of the world, thanks to a set of multilateral institutions, alliances and conventions that underpinned the western liberal system and the US’s place within it.

If Trump is serious in what he has said in the past 18 months – a big if, admittedly, for an arch-pragmatist apparently untethered by conviction or policy know-how – that global system looks very different today than it did on Friday morning. The US is now led by a man who, by his own word, believes the transatlantic alliance is obsolete, rails against free trade, opposes closer European integration, proposes to move his embassy in Israel to Jerusalem, calls refugees “illegals”, questions the US military guarantee for South Korea in the event of an attack from the North and speaks more warmly of the Russian president than any other world leader.

Not every Trumpian foreign policy idea is all that new, of course. His Syria policy (“bomb the hell” out of Islamic State and help the Kurds) is, when the bluster is stripped away, close enough to current US strategy. Barack Obama also came to power seeking a fresh start in relations with Moscow. Several presidential candidates spoke of moving the Israel embassy to Jerusalem only to shelve the idea once they entered the White House.

Abrupt departure

By any measure, however, Trump’s campaign rhetoric signals an abrupt departure. For some world leaders, this is cause for celebration. Syrian president Bashar al-Assad seemingly has little to fear from a US president who has not made his departure a condition of any peace deal to end the long-running civil war. Vladimir Putin, having seen his intervention on Assad’s side tip the balance of that conflict in his favour, in turn burnishing his own status as a global power-broker, has now seen the EU weakened by Brexit and pro-Kremlin politicians in Europe either win power (Bulgaria, Moldova) or move closer to it (Marine Le Pen and François Fillon in France).

Now Putin’s opposite number in the White House is a man who won the presidency – helped by a Russian intelligence operation, according to the US government – while extolling him as a great leader worthy of the US’s respect.

Little wonder that sentiment among US allies in eastern Europe, Asia, the Middle East and elsewhere veers between trepidation and alarm.

Yet some big questions remain unanswered. Given the absence of a political track record and his propensity to take both sides of the same argument, Trump is in effect a tabula rasa – a blank slate on to which his supporters can project their hopes and his opponents their fears. The sense that he has few fixed ideological positions comforts many allies, who believe that once they manage to get a word in his ear and he begins to feel the weight of his new responsibilities on his shoulders he will rein in his wilder inclinations.

Already there have been some shifts. During the campaign, for example, Trump spoke of withdrawing from the Paris climate deal. More recently he said he had yet to make up his mind on the issue. He has given mixed messages on the Iran nuclear deal, and has in recent weeks walked back the threat to abandon the decades-old pledge to defend South Korea from attack.

Competing power centres

Much depends on those around Trump. The triumvirate of Rex Tillerson as secretary of state, James Mattis at the Pentagon and Mike Flynn as the national security adviser contains a relatively wide range of opinion, and with it the potential for conflict. Others, not least Trump’s son-in-law Jared Kushner and chief strategist Steve Bannon, could be even more influential. The worldview of the Trump administration – and by extension the answers to some of the major global questions of the next four years – will take shape largely through the chaotic interaction of the various, competing power centres with which Trump has surrounded himself.

Barack Obama was ridiculed by his opponents when it emerged that in private he remarked that the first task of an American president in the post-Bush international sphere was: “Don’t do stupid shit.” For Obama that was a baseline. For many of Washington’s allies, it’s their highest hope for the Trump administration.