Across two continents, across two days, world leaders gathered.

On Monday, in London, they joined Britons (and viewers worldwide) in affectionately remembering Queen Elizabeth II, whose historic 70-year reign spanned the twilight of the British Empire.

On Tuesday and for the rest of the week in New York, the sentiments weren't as affectionate for a modern-day imperialist, Russian President Vladimir Putin, whose illegal, immoral invasion of Ukraine became the focus of scores of speeches to the United Nations General Assembly.

Several implied that imperialism was in fact at the heart of Putin's heartless brutality.

"Let us speak plainly," President Joe Biden said. "A permanent member of the United Nations Security Council invaded its neighbor, attempted to erase a sovereign state off the map. Russia has shamelessly violated the core tenets of the United Nations Charter — [none] more important than the clear prohibition against taking territory of their neighbor by force."

Pivoting from his usual, useful geopolitical framework of an existential struggle between democracy and autocracy, Biden homed in on sovereignty, which even most autocrats automatically defend. "If nations can pursue their imperial ambitions without consequences," Biden said, "then we put at risk everything this institution stands for. Everything."

French President Emmanuel Macron reflected the theme of imperialism, too. And not just regarding Russia, but for fence-sitters, when he said: "Those who are keeping silent today are, in a way, complicit in the cause of a new imperialism."

The president representing the victims of this imperialism, Ukraine's Volodymyr Zelenskyy, told the delegates in rejecting Russian aggression that "the U.N. Charter proclaims the equality of nations, and we proved that Ukraine is equal among the equals."

And then some. Zelenskyy, who evokes another midcentury Brit, Winston Churchill, spoke not from New York but from Kyiv by satellite, which required a special U.N. vote. Ukraine prevailed — as it has lately on the battlefield — by a 101-7 margin, with only the leaders of North Korea, Cuba, Belarus, Eritrea, Nicaragua, and Syria sticking with its fellow oppressor Putin.

Ominously, the international isolation isn't as potent as Putin's domestic critics are. No, unfortunately, not the brave antiwar Russians — the Kremlin's crackdown on them endures. But rather the far-right militarists increasingly critiquing the war (or in Russia, "special military operation"), especially after Ukraine's stunning counteroffensive success.

Accordingly, after being humiliated militarily in Ukraine and diplomatically in meetings with Chinese President Xi Jinping and Indian Prime Minister Narendra Modi, Putin tripled down with a call-up of 300,000 reservists (and maybe many more), a plan for sham referendums on the national status of territories taken in Ukraine, and a thinly veiled threat to use nuclear weapons.

The war and the resulting ramifications in geopolitics, energy and food distribution and the global economy have jarred an already unmoored world. Indeed, it's an "Age of Uncertainty," according to the centennial issue of Foreign Affairs, whose co-executive editor, Justin Vogt, said: "What's striking to us, and part of what we were trying to explore, was the degree of what I would call analytic uncertainty."

And one of these uncertainties is the outcome in Ukraine. More certain is that the eventual victor will shape this uncertain age, as strongly argued in one of the issue's essays from Yale historian Timothy Snyder.

"A Ukrainian victory," Snyder writes, "would confirm the principle of self-rule, allow the integration of Europe to proceed, and empower people of goodwill to return reinvigorated to other global challenges. A Russian victory, by contrast, would extend genocidal policies in Ukraine, subordinate Europeans, and render any vision of a geopolitical vision of a European Union obsolete."

For most of the postwar era, "imperial projects were seen to be something of the past," said Vogt, who added that the era was marked by "decolonization, independence movements, the rise of autonomous nation-states." Russia's invasion "has thrown us back into a much earlier moment when state behavior is less about norms and institutions and rules and just kind of sheer imperial grab for territory and power."

Jeffrey Mankoff, a senior associate with the Center for Strategic and International Studies' Russia and Eurasia Program, whose books include "Empires of Eurasia: How Imperial Legacies Shape International Security," said Russia is "much closer to its imperial past, in the sense that we can say the Soviet Union was basically an imperial state. And so efforts to build something post-imperial in Russia really only date to 1991. And many of the people who are in charge in Russia are products of the imperial system."

This includes Putin, whom Mankoff said was stationed as a KGB agent in one of the U.S.S.R.'s "colonies" — East Germany. "Moreover," Mankoff continued, "because Russia is a land power, effectively, and its colonies were geographically contiguous, it remains entangled with them to a much greater degree than the U.K. would be with India." Putin, Mankoff added, "made some kind of imperial restoration really kind of central to how he wants to build his legacy."

Such a restoration would reverberate globally. So would stopping it.

John Herbst, senior director of the Atlantic Council's Eurasia Center and a former U.S. ambassador to Ukraine, said that if Kyiv prevails, "Putin's revisionist policy, not just in Ukraine, but to all of the former Soviet space, which includes our three NATO allies Estonia, Latvia, and Lithuania, goes kaput. It's also a huge blow to the authoritarian moment that seems to be ascendant as recently as February, when Putin met Xi [Jinping] before the big invasion. I think the Chinese have been chastened by Putin's troubles in Ukraine."

If those troubles compound, Herbst "would guess it's going to prove to be the last gasp of Russian imperialism. Because he's losing this war, he will lose it, unless somehow the West goes wobbly."

It must not, lest it risk Russia's revanchism spurring other countries' imperial ambitions.

What Russia and other some other post-imperial states are trying to do, Mankoff said, is build a "world safe for empire" that "allows them to have sovereignty but doesn't respect the sovereignty of smaller state ... . And I think if Russia is successful in Ukraine it's not going to be the only instance of this kind of imperialism in the 21st Century."

Belying the famous phrase, the 20th Century saw that the sun did indeed set on the British Empire. While the U.K. relinquished much of its hard power across the world, it did retain a high degree of soft power, in no small part because of Elizabeth's gentle, genteel ways during this dusk.

Putin's ways are anything but. And with Western resolve and Ukrainian courage his dream of a new dawn of Russian empire will be eclipsed by an age of more certainty.