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

(CNN) Vladimir Putin waited a long time to get his face-to-face meeting with Donald Trump. But when it came last month at the G20 summit in Germany, it appears to have been worth his while.

What was going to be a talk on the sidelines of the G20 became a full-fledged bilateral meetingthat blossomed into a 2-hour-plus confab -- far longer than the 45 minutes finally penciled in.

The meeting wasn't just notable for its length: It was held in near secrecy.

Unlike Trump's sit-down meetings with other leaders such as Xi Jinping of China, where plenty of White House experts were at the table, only Secretary of State Rex Tillerson accompanied Trump to this cozy chat with Putin, who was aided only by his foreign minister, Sergey Lavrov.

But that wasn't Putin's only bite of the White House cherry at the G20. At a dinner for leaders and partners, Trump reportedly gave Putin a nearly hourlong, one-on-one meeting.

Aided only by Putin's translator, the circle of trust shrank even further than the bilateral meeting -- and with it the likelihood of ever knowing what was actually said

But whatever was said turns out to have been enough to convince Putin he's going to have to cut his losses with Trump.

The early anticipation, on both sides, that the two men and two countries could get along and improve relations appears to have evaporated.

Putin has never been asked why he believed Trump would act any differently than any other US president: country first, self second.

We know Putin did believe that Trump had ambitions to rewrite modern history, allowing the eagle and the bear to embrace. Why else would he have tolerated Russia's Kremlin controlled-media fawning over Trump's presidential victory?

And now, more than half a year later, Putin has finally had the chance to stare Trump in the eye and decide once and for all if all his birthdays and Christmases had indeed come at once.

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Whatever Putin learned from talking to Trump, he seems to have calculated that Trump is unlikely to be useful to him.

If that is the case, then Putin -- despite everything -- will be the first world leader to give up totally on this US President.

His calculation has quickly proved to be correct.

Much as Putin could still use a friend at the White House to help loosen international restrictions, ease his economy back to full health and rehabilitate Russia's international bully-boy image, his decision to crimp America's diplomatic mission in Russia -- cutting employees by a record 755 -- implies a "realpolitik" worthy of his Soviet predecessors.

The Russian leader appears to have concluded he needs to get tough, drop the honey and go back to the more familiar vinegar approach.

Having met and talked at length with Trump, Putin will have done what most KGB officers were trained to do: Spot weakness and speedily assess if it can be exploited.

Putin's judgment seems to be that Trump's weakness begins at home and that -- whether the US President likes it or not -- he is unlikely to come out of many of his domestic battles on top.

Any glimmer of hope that Putin may have had that the US leader might still be a useful partner in the immediate term has surely gone now that Trump has signed the sanctions bill that Congress passed last week.

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Whether Trump was outsmarted by Congress or finally caved to pressure doesn't matter in Russia. What matters is that Putin's got the measure of Trump.

The scene is now set for what longtime Kremlin and Putin critic Bill Browder told me late last year would happen between the two men: "We'll end up in a position where both these guys will be thumping their chests and staring each other down."

And here we are. Once these mission staffing cuts kick in in a month, Trump will be faced with a decision to follow usual protocol and make tit-for-tat Russian expulsions.

The stage is being set for Trump's first big overseas fight to be with the one world leader he won't criticize. Both men have resisted, but now the gloves are coming off -- and it's Putin who has landed the first punch.

In Trump's school of hard knocks, that's about as disloyal as it gets. However he frames it to himself, Trump will likely have a hard time not feeling some of this is personal.

While bigger issues such as North Korean missiles and Iranian nuclear plans loom on the horizon, none has become personal for Trump yet. From the get-go Russia has been the opposite.

Ultra-sensitive to the impression he was unfairly -- even unlawfully -- elected with Russian connivance, the wellspring of anger that bubbles below any Russia issue risks being exceptionally deep and potentially volatile.

If Trump decides to treat overseas leaders the same way he treats his own hires who have transgressed -- such as Attorney General Jeff Sessions -- then the tempo of the Russia relationship may accelerate.

As Tillerson said this week, Washington's relationship with the Kremlin is under strain: "The question, I think, of the events of the last week or so is -- Is it getting worse or can we maintain some level of stability in that relationship, and continue to find ways to address areas of mutual interest and ways in which we can deal with our differences without those becoming open conflicts as well?"

Mutual concern being Syria, Ukraine and election meddling.

Tillerson laid out the red lines for engagement on Syria: that Bashar al-Assad must go and so must Iranian forces. On Ukraine he said Russia must make good on its commitments to a ceasefire.

Vice President Mike Pence, currently on a tour of NATO Baltic state allies, this week gave insight to White House thinking on Russia: "No threat looms larger in the Baltic states than the specter of aggression from your unpredictable neighbor to the east.

"At this very moment, Russia continues to seek to redraw international borders by force, undermine the democracies of sovereign nations and divide the free nations of Europe against one another."

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Couple these frustrations with personal slight and maybe Browder is right: A showdown is coming, although Tillerson and Pence both say the US preference is for a constructive relationship with Russia.

Putin can hardly be happy with the new sanctions, but he will have proved himself right: that Trump is in trouble at home and may not be able to fight himself out of it. And if Trump is distracted at home, that can only create opportunities for Putin to outmaneuver the United States elsewhere.