Image copyright Getty Images Image caption The New York neighbourhood known as Turtle Bay is slowly being sequestered from the rest of Manhattan ahead of the UN General Assembly

Patrol boats are darting up and down the East River. The NYPD coffee and donut truck will soon be parked on 42nd Street.

The New York neighbourhood known as Turtle Bay is slowly being sequestered from the rest of Manhattan, the temporary border marked by concrete bollards, metal railings and a small army of police officers.

The annual carnival of global diplomacy is about to begin.

The United Nations General Assembly. Or UNGA, as it is known within an organisation addicted to acronyms. Or #UNGA, to use the hashtag hieroglyphics of this social media age.

Not diplomacy by Twitter, but the face-to-face real thing.

Some 84 heads of state and 44 heads of government are descending on New York, once again demonstrating the unique convening power of this embattled global institution.

Inevitably, though, the week will be dominated by the return of the former New York property tycoon who made his name a few blocks away on Fifth Avenue, and whose pre-presidential relationship with the UN was centred on bidding for the billion-dollar contract to renovate its headquarters.

Image copyright Reuters Image caption UN Secretary General Guterres (left) has been careful to avoid a public breach with the Trump administration.

Donald Trump is making his second appearance at UNGA, and this year he will not only have the platform of the speaker's dais in the general assembly - standing in front of green marble tiles he once derided as "cheap" - but also the famed horseshoe table of the Security Council.

By a fluke of the calendar, the United States this month holds the rotating presidency of that body, and on Wednesday Donald Trump will chair a meeting.

Initially, the Trump administration wanted the session to focus solely on Iran.

That, however, carried the risk of shining a spotlight on America's isolation on the Security Council over the Iranian nuclear deal.

So the agenda has been broadened to discuss the more generalised topic of "proliferation of weapons of mass destruction", presumably to avoid other signatories, such as Britain and France, from openly criticising the Trump administration's withdrawal from a deal which the council voted unanimously in 2015 to codify.

Perhaps that memo was swiped off the president's desk. Or if it has he's disregarding it. "I will Chair the United Nations Security Council meeting on Iran next week!" he tweeted on Friday.

Usually, open meetings at the Security Council are heavily scripted.

The president of the council is also expected to adhere to some mind-numbingly dull, formal wording. So it will be intriguing to see just how much this norm-busting, freewheeling president will behave.

Image copyright Getty Images Image caption This will be Donald Trump's second General Assembly

Certainly, the chamber has a cinematic feel, and is probably the closest thing in international diplomacy to a corporate boardroom.

In other words, it is precisely the sort of stage he relishes.

Yet he tends to be more respectful of protocol and norms in settings freighted with history, as he's shown previously at the Vatican and Windsor Castle. Judging by his Twitter feed, it sounds like he is in combative mood.

In his address to the General Assembly, it is hard to imagine Mr Trump using more incendiary language than he deployed last year.

Making his UN debut, he threatened to wipe a member state, North Korea, off the map. In what sounded in parts like a live performance of his Twitter feed, he also mocked Kim Jong-un as "Rocket Man".

Back then, war on the Korean peninsula seemed a real fire-and-fury possibility.

Twelve months on, however, relations between Washington and Pyongyang have thawed dramatically because of the Singapore summit between Mr Trump and his North Korean counterpart.

Image copyright Getty Images Image caption There's been a dramatic change since Mr Trump was threatening to wipe North Korea off the map

Instead, his most blistering rhetoric could end up being aimed at Tehran.

He will peer out over a multi-national audience more sceptical than it was last year.

The Trump administration's decision to move the US embassy in Israel from Tel Aviv to Jerusalem drew an acid shower of criticism from member states at the UN.

There is strong support for the Iranian nuclear deal. His withdrawal from the Paris climate change deal still rankles. The trade war with China is intensifying, and he's still in dispute with the EU and Canada.

Trump has berated Nato allies over burden sharing, and refused even to sign a joint communiqué at end of the G7 in Quebec City. For many listeners in the hall, America First means America alone.

What will President Trump say about the United Nations itself and the rules-based international order it was designed to institutionalise?

The dark joke in Turtle Bay is that the UN's most important peacekeeping mission right now is focused on the White House.

That mission, while yielding results early on, has run into trouble.

Though Donald Trump tends to avoid Twitter tirades against the UN - it doesn't get the Nato or World Trade Organization treatment - his administration has struck repeated blows against the organisation that America was instrumental in founding and provides 22% of the funding for.

It has withdrawn from the Geneva-based UN Human Rights Council, Unesco and the Paris climate change agreement.

It has cut funding for the UN's Palestinian relief agency (UNRWA), the UN Population Fund and for some peacekeeping missions.

It has refused to sign a migration compact, recently threatened the International Criminal Court with sanctions, and regularly complains of what it sees as the UN's anti-Israel bias.

President Trump is expected to emphasise US sovereignty during his speech and reaffirm his country will not be bound by UN mandates.

"It's always going to be the will of the American people not the will of the international community," explained the US ambassador Nikki Haley, as she previewed the speech.

This, then, could be a defiantly America First address from a president who has been more determined during his second year in charge to follow his own gut, nationalistic instincts on issues such as trade and Iran.

For all that, fears early on that his presidency posed an existential threat to the UN have turned out to be overblown.

Image copyright Getty Images Image caption Ambassador Nikki Haley has a different world view from her boss

The Trumpian wrecking ball has not wrought as much damage as many diplomats here first feared.

Nikki Haley, who is more of an internationalist, has mitigated much of the potential damage and even America First diehards concede that in some areas global co-operation is unavoidable given the transnational nature of certain problems.

That explains Mr Trump's first event at this year's UNGA, a meeting he is hosting on the global fight against narcotics with more than 120 countries in attendance.

Even unilateralists need multilateral partners, especially on issues such as the proliferation of opioids that are of enormous concern to American voters.

The UN Secretary General Antonio Guterres, the former Portuguese prime minister, is often seen as a more effective politician than diplomat, and he's been careful to avoid a public breach with the Trump administration.

During his first year in office, it was almost as if the former socialist leader had taken a Trappist vow of silence when it came to dealing with the billionaire president.

Early on, he also forged a strong working relationship with Nikki Haley, whom he viewed as a fellow defender of the international system.

However, Mr Guterres seems to be finding it harder to suppress his true feelings.

Media playback is unsupported on your device Media caption What the world thinks of Trump

In a recent interview with the Atlantic magazine, he claimed the soft power of the United States was "being reduced at the present moment", although, as ever, he avoided pointing the finger directly at the US president.

Mr Trump will not monopolise all the attention at UNGA. But the absence of leaders such as Vladimir Putin, Xi Jinping, Narendra Modi and even Pakistan's new Prime Minister, Imran Khan, further elevates his VIP status.

Rumours have abounded that Kim Jong-un will belatedly crash the New York party, one of the few things that could possibly upstage Mr Trump. But this speculation is more a case of journalistic wishful thinking.

Will Trump be Trump? Or will this America First president be more respectful of diplomatic norms in the home of multilaterism?

Be sure to tune in this week to find out.

Nikki Haley predicts the biggest TV ratings for a UN Security Council ever. That will be music to her boss's ears.