Image copyright Getty Images Image caption International leaders failed to get Mr Trump to recommit to the Paris climate agreement

We'll get to the G20 in a moment. But let's start with organisational psychology. There are some groups or teams that are greater than the sum of their parts, and there are others - that for all the talent and power they may have individually - are less.

Think soccer. Think last year's European Championship. Then tiny little Iceland were the heroes of the competition - and not just because of their wonderful fans.

This team of part-timers played way above their ranking, there was solidarity, singleness of purpose, real determination. And think of England. Absurdly well paid and pampered players, whose showcase is the Premier League, the wealthiest in the world - and they were just a bunch of inked and primped individuals who seemed to have no team spirit. Dire. Just dire.

And so to the G20. It is not lousy like the England team were - I mean nothing could be as bad as that. But it is less than the sum of its parts.

What of the joint communiqué (that will have been toiled over by bureaucratic draftsmen for days to find a bland enough form of words that all the leaders can sign up to) will be remembered in six months' time? What ringing declaration will change lives? What plan of action will we look back on and think "turning point"?

Image copyright AFP Image caption Police used water cannon to break up protests

No, I suspect this G20 will be remembered for two things.

The first, the appalling violence on the streets of Hamburg from a sizeable group of protestors who seemed hellbent on burning and looting (there was something surreal on Friday night, drinking a beer in the 20th floor bar of our hotel and watching the fires and the police on the street below - as if it was a rioting son et lumière spectacle put on for our entertainment).

And second - the first meeting between Russian President Vladimir Putin and US President Donald Trump.

I suspect if you asked Mr Trump what was most memorable for him, he too would say the Putin meeting - but also his day in Warsaw, when thousands of Poles chanted his name and "U-S-A". Remember the crowd does for him what a battery station does for a Tesla car.

His speech on the challenges facing the West was well crafted with an argument to make. And he just sounded - well - more committed to the idea of the "West" than he did when he was last in Europe at the Nato summit and the G7.

His definition of what constitutes Western values may not have been to everyone's tastes - but he also committed the US to Article 5 of Nato, with its pledge that an attack on one is an attack against all. And for those countries who are part of the Alliance and abut Russia, they'll have been reassured by that.

Which brings us to the meeting with the Russian leader. I've read analysis since suggesting that it was a big victory for Trump; and some arguing that Mr Putin came out of top.

But that is to view the encounter as a zero sum game. Maybe both men emerged feeling they had won. Putin was treated seriously, wasn't lectured on human rights and democracy (his regular complaint about what it was like dealing with Barack Obama), and the meeting went on for nearly two hours more than its allotted time.

Image copyright Reuters Image caption Was the first meeting between the two presidents a victory for Mr Putin or Mr Trump?

Why does that matter? Because the Russian leader will feel he was treated as an equal to the US president with major things to discuss. Mr Trump will feel he has put the "Russia controversy" behind him. He raised it with Mr Putin, right, let's move on.

But that might be wishful thinking. Particularly after the Russians disclosed that the American president accepted the assurances of Mr Putin that Moscow hadn't interfered in last November's election.

The White House has not challenged that. And that leaves a very simple and astonishing point. Given the choice between accepting what his intelligence agencies have told him, and what Vladimir Putin has told him to be true, he's gone with the Russian.

And that might not play well in Washington, where there are myriad inquiries going on into the Trump campaign's links to Russia - and a Republican Party, who by and large given the same choice over who to believe, will go for the nation's intelligence professionals. Donald J Trump might want a reset in the relationship with Russia, but there are political constraints on him in Washington.

And then there are the decisions of the G20 itself. On two of the big ticket items - trade and climate change - it was really the G19 plus America. Mr Trump was intransigent in the face of appeals by other nations to recommit to the Paris climate change agreement. And there are real fears among European leaders that the president could unleash a trade war, over his insistence that America is being ripped off.

So if you're marking Mr Trump's card on how he got on, it really depends on where you stand on those issues. And as for the G20, it's hard to be more than the sum of its parts, when the biggest and most economically powerful member of the orchestra is humming a different tune.