MUNICH (NYTIMES) - When US President Joe Biden declared last Friday (Feb 18) that he was convinced Russian President Vladimir Putin had decided to attack Ukraine "in the coming week, in the coming days", the sceptics among American allies suddenly fell quiet.

Hours before, Mr Biden had informed them that US intelligence agencies had just learnt that the Kremlin had given the order for Russian military units to proceed with an invasion.

Now the debate has shifted to how Mr Putin will do it: in one massive nationwide attack; a series of bites that dismantle the country, piece by piece; or a python-like squeeze.

That last option is made all the easier with the news on Sunday morning that Belarus is allowing Russian troops to remain indefinitely, where they can menace Kyiv, the Ukrainian capital.

Mr Putin might be betting that he can shatter Ukraine's economy and oust its government without having to immediately roll in tanks.

His strategic choices over the next few weeks may make a huge difference in how the world reacts.

If he strikes to take the whole country in a single blow - the approach that senior US military and intelligence officials and many outside analysts now think is the most likely - it could provoke the largest, most violent battle for European territory since the Nazi surrender in 1945.

There is little question that the full package of sanctions and technology export cut-offs would be invoked almost immediately.

International condemnation would follow, although Mr Putin may be betting that it would not last long, and that the world would gradually get accustomed to a new, larger Russia reconstituting the sphere of influence that was once the hallmark of the old Soviet Union.

"Everything leading up to the actual invasion appears to be taking place," Secretary of State Antony Blinken said on Sunday on CNN.

Yet, he held open the possibility of a last-minute diplomatic solution.

French President Emmanuel Macron said both the US and Russia have agreed to a summit in the coming days.

The White House confirmed Mr Biden had agreed to the meeting with Mr Putin in principle, provided Russia does not invade Ukraine.

The information passed to Mr Biden from the intelligence agencies left unclear whether Mr Putin's orders would lead to a massive invasion or a more gradual approach that would give the Russian leader more opportunities to exploit fissures just beneath the surface in the Western alliance arrayed against him.

He could, for example, test the proposition that Germany or Italy, the two Western European countries most dependent on Russian-provided gas, might falter in their resolve.

Those were the scenarios being discussed most intensely on the weekend at the Munich Security Conference, the annual meeting of government ministers, corporate leaders and strategists, where attendees gamed out Mr Putin's choices.

"If he is intent on escalating, I don't think it's a sudden blitzkrieg to Kyiv and the ouster of the... government," said Dr Ian Bremmer, president of the Eurasia Group, a geopolitical consulting firm.

"It's much more likely to look like a recognition of the independence of the breakaway territory" around Luhansk, in the east.

"You hope, if you are Putin, that leads to more skittishness of some of the Nato allies, less alignment with Nato, more opportunities for Russia to get what it wants without having to go full scale into Ukraine," Dr Bremmer said.

A few weeks ago, some US officials shared that sentiment.

Mr Putin, they noted, presumably wanted to achieve his goal - a halt to Ukraine's drift towards the West - as cheaply and with as few casualties as possible.

All he sought was a friendly, pliable government like the one he has in Belarus, said one senior US official, who spoke on condition of anonymity because of the ongoing diplomacy.

Belarus President Alexander Lukashenko has tied the security of his country to the presence of the Russian military.

"They will be here as long as necessary," said Mr Lukashenko, who is considering inviting Russia to place its nuclear weapons back on Belarusian territory..

It would be, many suspect, a refinement of Russia's hybrid-warfare playbook.

"Putin has developed and demonstrated over a decade of aggressive action that he knows how to fine-tune grayscale warfare that is hard to attribute,'' said Senator Chris Coons, who is close to Mr Biden.

"We saw it in Crimea, the combination of covert and overt actions to interfere with and undermine a democratic election," he added.

"But this is a bit different. It's not hard to figure out what nation these 150,000 troops have come from. And that's why I don't think that a lesser invasion - a 'minor incursion' if you want to call it that-- would result in a lesser penalty.

"We're not in a place anymore where proportionality is a key piece of the argument."

Mr Biden briefly floated the phrase "minor incursion" in January at a news conference. At the time, he suggested that the allies might not impose full sanctions for a modest expansion of the territory Russia already controls around Crimea.

In that case, Mr Putin might seek to test the international reaction to each step - seeing what kind of punishment, or military resistance, he might face.

But almost as soon as the words were out of Mr Biden's mouth, White House officials walked them back. The next day, the President declared that any move over the border - no matter how minor - would trigger the full sanctions package.

Still, officials in the Biden administration are discussing with some urgency how the US might respond to a series of smaller, or less visible, steps by Russia.

Russia could also cripple the Ukrainian power grid and communications systems.

Mr Biden recently sent the deputy national security adviser for cyber and emerging technologies, Ms Anne Neuberger, to brief Nato on what that might look like - and for the possibility that the cyber attacks could spread to Western Europe and the United States.

Another "minor incursion" might be paramilitary activity, or a prelude to a traditional invasion reminiscent of the Soviet Union's invasion of Hungary in 1956 or Czechoslovakia in 1968.

But over the past two weeks, administration officials have publicly shifted their assessment, saying they think Putin is likely to go big.

About three weeks ago, US intelligence officials picked up more and more evidence that the primary target was Kyiv, a prediction supported by the massing of new troops on the Belarus-Ukraine border, a mere 160km or so from the Ukrainian capital.

Whether those troops would just menace the capital from afar, raining rocket attacks on it, or whether the Russian plan is to ring the capital city with troops but not enter it, to avoid urban warfare, is unclear.

But in briefings to members of Congress and others, the Pentagon and US intelligence officials have described a worst-case scenario that they now consider to be likely: A week or two of terror, constant rocket attacks and street fighting and, ultimately, a hunt for anyone who supported the democratically elected government of President Volodymyr Zelensky.

Mr Blinken gave a somewhat sanitised version of what that might look like in a speech to the United Nations last Thursday. But the more granular assessments suggest Russia would begin by cutting Ukraine's Internet connections to the outside world, jamming cell and computer networks and frying the communications among Ukrainian military units.

Then would come salvos of ballistic missiles, which can already be seen on mobile launchers moved to the Russian and Belarusian borders with Ukraine.

US officials who have had access to some of the Russian planning - they are discreet about how they have obtained it - say it calls for overwhelmingly intense fire.

"We were told to expect tens of thousands of casualties in the opening days," said one senior official who has received the briefing, speaking on condition of anonymity to discuss the intelligence.

Ukraine's military, far better equipped and trained than it was eight years ago when Russia surprised the world by taking Crimea, would fight back hard, most officials expect. Nato would rush in supplies. The fighting could last weeks, officials were told, before settling into a guerilla war.

But some intelligence assessments suggest that after that intense fight and installing a puppet government, the Russians might withdraw, to avoid an occupation and the resulting insurgency.

Several of Mr Biden's senior advisers said in recent days they were sceptical that such a withdrawal would happen, suggesting that would only lead to eventual uprisings against the government - the kind that took place on the Maidan in Kyiv, also known as Independence Square, exactly eight years ago this weekend.

Hundreds of thousands of Ukrainians took to the streets there in 2014 and ousted President Viktor Yanukovych, who fled to Russia.

Mr Putin remembers those events well.

They have led, in many ways, to this day, and this crisis. The US assessment is that he is determined not to let street protesters interfere with his strategy to control the country, and the region, for a second time.