The G-20 quickly turned into the G-19, with President Trump, the famous self-proclaimed dealmaker, isolated on the sidelines as the rest of them made their deals without him.

Remember Trump’s slogan: “Make America Great Again”? For 70 years, since the end of World War II, America has been great indeed: it’s been the leader of the Free World, perhaps a trite-sounding phrase since we’ve heard it so often. But undeniably true, and something that old Free World — when you consider the nuclear-armed Soviet Union and its Eastern European captive allies — was delighted to have.

No longer. America’s greatness a la Trump seems predicated on our retreating back into a form of isolationism that our strength, and geographic position, makes possible — as it did in the lead-up to both World Wars.

Climate change? A hoax. Why should we cooperate with the other 19 strongest economies in the world on global warming?

Trade? Who needs all those multilateral deals like the one the EU just negotiated with Japan? We’re the United States, the strongest economy in the world.

Globalization? That’s for the rest of the world.

No, the US interest, says Trump, is in setting up a new world-trade order, initiated with US tariffs on steel imports, which would undoubtedly set off a global free-for-all on import taxes and further weaken the US economy — and, indeed, that of the industrial world overall. Russia, with its raw material–based economy, would be a relative winner.

Trump the dealmaker? The would-be emperor, I’m afraid, has no clothes. Buying and selling Manhattan real estate, it turns out, is a little bit easier — and takes a lot less sophistication — than dealing with world leaders, even when most of those leaders run countries that are close allies of the US.

Trump at the G-20 was conspicuously the odd-man out. Most damning are the pictures of Trump sitting alone at the conference, or standing by himself, surrounded by colleagues in groups of two or three actively pursuing their joint interests.

Trump’s most self-satisfying accomplishment in Hamburg was to turn a scheduled 30-minute meeting with Russia’s leader, Vladimir Putin, into a two-hour-plus marathon.

Trump has this weird respect — fear? jealousy? — for the Russian strongman. Does he admire Putin’s physical exploits, those bare-chested horseback rides? And why not: a picture of a bare-chested Trump, his overweight body sagging on the back of some poor horse, would be banned in most countries as obscene.

The key US intelligence agencies have confirmed, beyond any reasonable doubt, that Russia tried to hack into Hillary Clinton’s emails during the presidential campaign in an effort to help Trump — a dangerous attack on one of the fundamental bases of our democracy.

So, in his tete-a-tete with Putin, how does Trump deal with the Russian hacking? According to the Russians, Trump dismissed the charges as “a campaign against Russia.’’ Secretary of State Tillerson, the only other American at the meeting, claimed there needed to be a “robust’’ exchange on the issue.

Who should we believe?  It shows how little trust Americans put in Trump that the question would even be raised. As the NY Times’ Maureen Dowd portrayed the confrontation — sarcastically but maybe not too inaccurately:

“Trump: ‘Did you do it?’ 

Putin: ‘Nyet.’ 

Trump: ‘Whew. Glad that’s out of the way. So let’s do a joint cybersecurity program and share our passwords.’ 

Putin: ‘Da.’’’

Perhaps some good did come out of his 150-minute Putin extravaganza. An agreement to work together to control the world’s most dangerous situation, a nuclear-armed North Korea? Nyet.

An agreement not to interfere in European or US elections? Nyet.

But a little credit, please: he got them to agree on that cybersecurity program. Yes: Russia hacks into the Democratic Party’s email system and Donald Trump joins with Putin on a cybersecurity deal. The “fox guarding the henhouse” comparison was the unsurprising Washington reaction. Republican Senator Lindsey Graham’s comment was perhaps the most damning: “It’s not the dumbest idea I have ever heard but it’s pretty close.” 

Even Trump can eventually see the light, at least when he’s blinded by it: before the sun had risen again, he announced he was backing out of his cybersecurity arrangement with buddy Vlad.

Post–G-20, it hasn’t gotten any better for Trump with the revelation that his son, his son-in-law, and his then-campaign manager met last year with a Russian lawyer to obtain negative information about Hillary Clinton. It’s understandable that Trump would have sought legitimate information to use in his campaign against Clinton. But from Russia?

Republican officials who had worked on the George W. Bush campaign were outspoken in their reaction: Sure, I handled opposition information, one noted, “but never ever from a foreign adversary.” And another Republican, noting that if “foreign interests offer you information, you call the FBI,’’ specifically praised the Gore campaign for having done just that when “it was sent the Bush debate briefing book.”

Trump, of course, denies any knowledge of the meeting. His campaign manager, together with Donald Trump Jr. and Jared Kushner, agree to meet with a Russian who has ties to the Kremlin — and they don’t bother to tell their boss, their father, their father-in-law? Trump’s new bar for straining credulity. 

As the Trump-Putin connection has grown more obvious, Trump’s attempts to hide it have only raised other questions.

Why make a private meeting with Putin the highlight of your G-20 experience?

Why initially buy into a cybersecurity deal with Russia?

Why did close Trump associates meet at least several times with Russian officials during the campaign — and then try to keep the meetings a secret?

Maybe Trump just has a natural affinity for Slavic men and women. Or maybe he’s basically a sentimentalist at heart, with warm memories of his 2013 Olympics in Russia.

Or, maybe, it’s not love or respect: it’s political blackmail. Maybe Putin really does have some embarrassing hold over Donald Trump. What else makes more sense?

Come on, Robert Mueller: speed up that investigation.