Brett Bruen is president of the Global Situation Room, Inc., an international consulting firm. He serves as an adjunct faculty member in crisis management at Georgetown University and on boards at Harvard University, University College Dublin and UNICEF. Under President Obama, he was director of global engagement at the White House and spent 12 years as a U.S. diplomat. View more opinion articles on CNN.

(CNN) Trump is a master of New York-style showmanship. The United Nations General Assembly , therefore, should be his finest hour: hosting world leaders in his town.

Instead, he is likely to be relegated to an irrelevant sideshow.

When the UNGA kicks off on Tuesday, the depths of America's descent will be on full display. Despite currently holding the presidency of the Security Council, the US will struggle to influence the course of events.

Whether on Iran, Israel or the International Criminal Court, the US will find few nations willing to lend their support to its polarizing positions.

Weary from his past attacks and antics , other world leaders would just as soon avoid crossing paths with President Trump. Where they do have to engage, it will be an exercise in managing a toxic substance. Go in protected and get out as quickly as possible. When he does inevitably lash out, they will do their best to contain the damage.

Bullying and bulldozing have brought tumbling down our traditional friendships and a firm foundation for international cooperation. In an unusual move, the Secretary-General of the UN will not address the Security Council session President Trump plans to chair.

We now stand alone on unsteady ground. Meanwhile new structures and strategies have been erected.

Our allies are building security, economic and political plans without us. We have entered a post-American era in which other nations will depend far less on our leadership. At the UN, we will witness the emergence of some of those new roles and relationships.

On the positive side, the likes of Europe, Canada and Japan are becoming increasingly assertive . Defending traditional values and a Western worldview. They will even find themselves sharing the same side as Russia and China on issues like the Iran nuclear deal and the Paris Accord.

More problematically, we have already seen regional powers trying to challenge past customs and constraints. Sensing an opening, Russia and China have fast forged a close alliance. Saudi Arabia put Western nations on notice that it will no longer tolerate even mild criticism of its human rights record. Turkey tried to avert a humanitarian crisis in Idlib, Syria, as we stood on the sidelines.

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For the first time in history, America will find itself consistently isolated on major issues before the Security Council. That's a role more commonly played by the likes of Russia or China. Sure, there were a few cases in the past when it had been in the minority on matters like Israel or Iraq. But, it has never struggled to rally a majority behind most of our diplomatic agenda.

Trump will likely try to pass off attacks on Iran as an effort to stem the spread of weapons of mass destruction. No one will be fooled by the thinly veiled attempt to undermine the Iran deal. It will represent an unprecedented rebuke of the United States. Even in the most difficult periods of the Iraq War, America still managed to hold together the Anglo alliance along with an array of other nations. But they are no longer playing ball.

Donald Trump has complained about the disproportionate costs borne by the US . He really won't like the expenses that await us. Maintaining international institutions will prove to have been a lot cheaper than managing more frequent global conflict. The assumption of their accession to our policies will be gone. We will now be forced to pay handsomely when we want to gain the backing of many nations.

Ironically, the tables will have turned or at least be more even. For years the US could force nations that wanted our support to make large weapons purchases or grant us unique concessions. No more. Especially after the trade wars, tweets and tough telephone calls, their price will be high, if not stratospheric.

Will North Korea offer the chance for a new beginning? Most leaders are pleased to at least see the verbal assaults abate and the missiles muzzled for the moment. Yet, Trump has proven a poor negotiator. The fear remains that he will lead the world into an even worse place.

Trump may well dominate the headlines out of the UNGA. As the photo of Chancellor Angela Merkel looming over him at the G7 summit illustrated so well, he is far less likely to dominate or direct the debate. His sideshow may prove to be a distraction, but it will do little to change the clear power shift taking place.

Some will see opportunity in America's absence. Others will act as though little has changed, perhaps hoping their old ally will return. Most will proceed with caution. We are entering the dawn of a new era where lines are being redrawn.

For now, we are unlikely to see the international community make much progress on major issues. They will remain too preoccupied with preserving what they can of the power and privileges they enjoyed under the old order.

More pressing will be efforts to prevent worst-case scenarios from coming to pass. It's a dangerous period for the world, one that has echos of the British Empire's descent in the beginning of the last century. Trump may indeed get a dramatic show, but the ending will be far different than he hoped.