Nic Robertson is CNN's international diplomatic editor. The opinions in this article belong to the author.

(CNN) Ever heard the one about the three world leaders who walked into a meeting room?

The first says: "I'm the most powerful man in the room -- 33 million people follow me on Twitter."

The second says: "I'm the most powerful man in the room -- I've banned Twitter."

And the third says: "No, I'm the most powerful man in the room -- I've got all the passwords."

The punch line, of course, is that no one can tell any of them that they are wrong.

Although we are hardly likely to hear Presidents Donald Trump, Recep Tayyip Erdogan and Vladimir Putin engage in such discourse at the G20 in Hamburg , it's not beyond reason to believe these men are capable of such thought.

While each is at a different stage of his leadership -- Putin has been at it for years, Erdogan cemented control of his a few months ago and Trump is a relative newbie on the world stage -- they are all currently emerging as great global disruptors.

The G20 summit in Hamburg comes at time of particular global disunity. Elder statesmen and women of diplomacy have declared that the post-WWII world order is over, potentially leaving decades of dependable stability in terminal decline.

As today's most powerful 20 leaders gather in Germany's northern port city, some among them may cast an eye across the city's skyline remembering when much here looked as Mosul does today: crumpled and broken by war.

In July 1943, allied bombers began an aerial bombing campaign, Operation Gomorrah. In just eight days, 42,000 civilians were killed and 37,000 were injured.

The awfulness of that war and the international desire to never again allow megalomaniacs to hold complete global sway is part of what makes the meeting of world leaders at the G20 today so important. It is, partly, why we should all care that some of the many grievances that these leaders will grudgingly discuss here do get resolved.

It doesn't help that some of those arriving in Hamburg toting the biggest bags of diplomatic gripes and animus are world leaders buried so far in their own beliefs that they've lost sight of the art of compromise.

JUST WATCHED Trump heads to summit amid disagreements with G20 nations Replay More Videos ... MUST WATCH Trump heads to summit amid disagreements with G20 nations 04:42

Take Putin. Did he ever say sorry for Russia's role in the US elections? Has he given Crimea back to Ukraine, or even considered ending its illegal annexation? Has he ever stopped to think that President Bashar al-Assad -- whom he backs in Syria's civil war -- might, after murdering hundreds of thousands of his own people, not be fit to lead the country any more?

And then we have Trump, whose own wife admits that when he's challenged he punches back 10 times harder

He comes to the G20 a global outlier on climate change. Shortly after the G7 summit in May, he snubbed the other leaders by dumping Barack Obama's commitment to the Paris climate accord.

And like Putin, Trump's disruptions are not singular. He has recently ratcheted up tension with China, courting controversy over weapon sales to Taiwan and testing waters around disputed islands -- not to mention his tough talk on trade.

All of that baggage seems to sink any hope of a meeting of minds on North Korea, something other G20 leaders were hoping might walk the region back from the risk of nuclear-tipped confrontation.

Of course this is exactly the impact North Korea's Kim Jong Un will have hoped for.

His test-launch of what Secretary of Sate Rex Tillerson described as an intercontinental ballistic missile not only raises the stakes of failure, but sows doubts that his nuclear ambitions can be stopped. Already his timely provocation has united Russia and China in Trump's face.

Trump arrives in Hamburg also at odds with his host, German Chancellor Angela Merkel , on trade and on climate change. How much they might agree on Russia, Syria and Ukraine is unclear, in part because until now Trump hasn't made clear his own position on any of these issues or what his long-term policy might look like.

They will both likely agree that the other outsized strongman attending -- Turkey's President Erdogan -- is an emerging problem child. Not only because he administered a referendum that delivered the powers of Parliament to his hands in the presidency, but also his penchant for riotous assembly.

Having seen how Erdogan's security team beat protestors when Erdogan visited Trump in Washington several weeks ago, Merkel has blocked Erdogan's request to hold rallies in Germany on the sidelines of the summit.

Trump's problems with Erdogan are bigger than Merkel's. As fighting in the Syrian war appears to be reaching its conclusion -- ISIS is losing its grip of its previous stronghold and de facto capital in Raqqa -- Trump's arming of Kurdish fighters to kill ISIS fighters angers Erdogan, who considers both ISIS and the Kurdish forces to be terrorists.

For his part, Putin is happy to have Trump and Erdogan squabble over this -- it allows his ally Assad to reap the rewards.

But beyond the fight for Raqqa, the real strategic end game in Syria is the race for the ISIS-riddled eastern town of Deir Ezzor. It'll be the hidden subtext of any G20 talk over Syria.

If Assad gets there first, then he, Russia and Iran control the vital highways that turn the Tehran, Baghdad, Damascus, Beirut axis into one smooth tarmac ride from the Persian mountains and Iran's mullahs to their seaside proxies Hezbollah, more than 1,000 miles away in Lebanon.

This outcome is unpalatable to G20 member Saudi Arabia. As King Salman has canceled his visit, the diplomatic stakes are already raised. His son, the young, ambitious and recently promoted Crown Prince Mohammed bin Salman, is likely to be far tougher to tame at the top table of global politics.

MBS, as he is known, arrives already exercised by tiny Gulf state Qatar's refusal to fall into line with his and the UAE's recent demands to cut its ties with terrorist groups, among other things. His peers at the summit will no doubt encourage the young prince to cool his jets and try to reach a deal.

This backdrop makes it unlikely that he will be in any sort of mood to compromise on Syria and see his nemesis Iran gain more traction in the region.

British Prime Minister Theresa May is up for making friends and getting deals done, but her clout is on the wane.

Yet it's not all doom and gloom. A rising international star, the fresh-faced French President Emmanuel Macron, can be expected to inject some optimism. He believes Trump can be brought in from the isolationism of "America First."

His first G20 could be his fellow europhile centrist Angela Merkel's last.

Germans go to the polls in the next few months. While she seems to enjoy something of a lead, not to mention the respect of many of the leaders, she could still use a positive summit to boost her popularity.

But even on this Trump is a disruptor.

He has talked Merkel down so much, criticizing her refugee policy and slamming German car sales to the US. With Merkel's decision to hold the G20 in a city center building that can be targeted by protestors, Trumps German critics will likely be heard.

Suffice to say, rarely in the field of recent global diplomacy have so many relied on so few for so much.

Therefore it's a shame, in a way, about the strongmen.

A few less of those might just have tipped the balance towards a more favourable G20 outcome.

Talking of which, did you hear about the three world leaders who left the meeting room? They all agreed that they were the best leaders at the meeting and had won all their arguments.