Is Russia preparing to invade Ukraine and what does Putin want?
Are Russian forces getting ready for war in Ukraine? At least 100,000 troops are positioned within reach of Ukraine's borders as Russia demands security guarantees from the West.
The US says Russia has 70% of the forces in place for a full-scale invasion, but Russia has repeatedly said it has no such plans. What happens next could jeopardise Europe's entire security structure.
Why is Russia threatening Ukraine?
Russia has long resisted Ukraine's move towards European institutions, and Nato in particular. Its core demand is for the West to guarantee Ukraine will not join Nato, a defensive alliance of 30 countries.
Ukraine shares borders with both the EU and Russia, but as a former Soviet republic it has deep social and cultural ties with Russia, and Russian is widely spoken there.
The threat is being taken seriously because Russia has invaded Ukraine before.
When Ukrainians deposed their pro-Russian president in early 2014, Russia annexed Ukraine's southern Crimean peninsula and backed separatists who captured large swathes of eastern Ukraine. The rebels have fought the Ukrainian military ever since in a conflict that has claimed more than 14,000 lives.
How big is the risk of invasion?
Russia says it has no plans to attack Ukraine: and armed forces chief Valery Gerasimov even denounced reports of an impending invasion as a lie.
Nato's secretary general warns the risk of conflict is very real. And while US officials have stressed they do not believe Russia has decided on an invasion, they think it could happen in the coming weeks.
President Biden's top military officer, Gen Mark Milley, has warned the scale of the Russian forces would cause a significant amount of casualties and fighting in urban areas would be horrific.
Ukraine is less convinced of the risk and its president has appealed to the West not to spread "panic". France believes Mr Putin's main aim is in gaining a better security deal.
Nevertheless, President Vladimir Putin has threatened "appropriate retaliatory military-technical measures" if what he calls the West's aggressive approach continues.
Thousands of Russian troops are taking part in exercises in Belarus, close to Ukraine's northern border, and Russia's deputy foreign minister recently compared the current situation to the 1962 Cuban missile crisis, when the US and Soviet Union came close to nuclear conflict.
What does Russia want from Nato?
Russia has spoken of a "moment of truth" in recasting its relationship with Nato. "For us it's absolutely mandatory to ensure Ukraine never, ever becomes a member of Nato," said Deputy Foreign Minister Sergei Ryabkov.
President Putin explained that if Ukraine joined Nato, the alliance might try to recapture Crimea.
Moscow accuses Nato countries of "pumping" Ukraine with weapons and the US of stoking tensions to contain Russia's development. Mr Putin has complained Russia has "nowhere further to retreat to - do they think we'll just sit idly by?"
In reality Russia wants Nato to return to its pre-1997 borders.
It demands no more eastward expansion and an end to Nato military activity in Eastern Europe. That would mean combat units being pulled out of Poland and the Baltic republics of Estonia, Latvia and Lithuania, and no missiles deployed in countries such as Poland and Romania.
In President Putin's eyes, the West promised back in 1990 that Nato would expand "not an inch to the east" but did so anyway.
That was before the collapse of the Soviet Union, however, so the promise made to then Soviet President Mikhail Gorbachev only referred to East Germany in the context of a reunified Germany.
Mr Gorbachev said later that "the topic of Nato expansion was never discussed" at the time.
Russia has also proposed a treaty with the US barring nuclear weapons from being deployed beyond their national territories.
What does Russia want with Ukraine?
Russia seized Crimea in 2014 arguing it had a historic claim to it. Ukraine was part of the Soviet Union, which collapsed in December 1991 and Mr Putin said it was the "disintegration of historical Russia".
A clue to President Putin's thinking on Ukraine came in a lengthy piece last year when he called Russians and Ukrainians "one nation". He labelled Ukraine's current leaders as running an "anti-Russian project".
Russia has also become frustrated that a 2015 Minsk peace deal for eastern Ukraine is far from being fulfilled.
There are still no arrangements for independently monitored elections in the separatist regions. Russia denies accusations that it is part of the lingering conflict.
Can Russian action be stopped?
President Putin has spoken several times to Mr Biden and France's Emmanuel Macron says the Russian leader promised him during marathon talks that he "would not be the source of an escalation".
Ukraine: How big is Russia's military build-up?
The question is how far Russia will go. President Biden has warned that a full-scale invasion would be a disaster for Russia. But if it was a minor incursion, he said controversially that the West would "end up having to fight about what to do".
The White House has stressed any move across the border constitutes a renewed invasion - but points out Russia has other weapons, including cyber-attacks and paramilitary tactics. When 70 Ukrainian government websites went down in January, Russia denied Ukrainian accusations that it was behind the attack.
The Pentagon has accused Russia of preparing a so-called false-flag operation, with operatives ready to release graphic video of a fake attack to provide a pretext for invasion. Russia has denied it.
Russia has also handed out more than 500,000 passports in rebel-run areas, so if it does not get what it wants then it could justify any action as protecting its own citizens.
However, if Russia's only aim is to force Nato away from its backyard, there is no sign of it succeeding.
Nato's 30 members have turned down flat any attempt to tie their hands for the future. "We will not allow anyone to slam closed Nato's open-door policy," said US Deputy Secretary of State Wendy Sherman.
Ukraine is looking for a clear timeline to join and Nato says Russia has "no veto, no right to interfere in that process".
And non-Nato members Sweden and Finland have also rejected Russia's attempt to stop them beefing up their ties with the alliance. "We will not let go of our room for manoeuvre," said Finland's prime minister.
How far will the West go for Ukraine?
The US and other Nato allies have made clear they have no plans to send combat troops to Ukraine itself but are instead offering support. The Pentagon has put 8,500 combat-ready troops on alert and is deploying 3,000 extra soldiers to Germany, Romania and Poland.
The main tools in the West's armoury appear to be sanctions and military aid in the form of advisers and weapons.
Poland has offered an array of surveillance drones, mortar bombs and portable air-defence systems. The UK, Denmark, Canada, the Czech Republic and Baltic republics have also offered security assistance.
President Biden has threatened Russia's leader with measures "like none he's ever seen" if Ukraine is attacked. So what would they involve?
The ultimate economic hit would be to disconnect Russia's banking system from the international Swift payment system. That has always been seen very much as a last resort, and there is concern it could badly impact the US and European economies.
Another key threat is to prevent the opening of Russia's Nord Stream 2 gas pipeline in Germany, and approval for that is currently being decided by Germany's energy regulator.
President Biden has also warned that he would consider personal sanctions on Vladimir Putin, if Russia invades Ukraine. The UK has also warned that "those in and around the Kremlin will have nowhere to hide".
What would a deal look like?
A potential agreement would have to cover both the war in eastern Ukraine and the wider security issue. After visiting Moscow, President Macron said talks would soon be revived and include Russia and Ukraine along with France and Germany - known as the Normandy quartet.
Ukraine is deeply unhappy about the terms of the Minsk peace agreements of 2014 and 2015, which it feels gave away too much to Russia and the separatists. "Like it or not, my beauty, you have to put up with it," were President Putin's words on the issue to Ukraine's leader.
An indication of where the West might be looking on a wider security deal with Russia could stem from a leaked document sent to Moscow by the US and Nato.
In it, the US says it could be willing to start talks on limiting short- and medium-range missiles as well as offering negotiations on a new Start treaty on intercontinental missiles.
Washington would also provide reassurance that it had no cruise missiles in Poland or Romania as part of a "transparency mechanism", with Russia providing reassurance on two Russian missiles bases.
https://www.bbc.co.uk/news/world-europe-56720589
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