Why is Russia threatening Ukraine? Boris Johnson joins last-ditch effort to prevent war

As the crisis enters a “critical juncture” Downing Street conceded Moscow could be planning to invade “at any moment”.

Mr Johnson will hold talks with world leaders before a trip to Europe during this week’s “window of opportunity” for de-escalation.

The UK and other Nato allies have urged their citizens to flee Ukraine and some airlines have cancelled flights to Kyiv amid growing concerns that the estimated 130,000 Russian troops amassed on the border could be poised to attack.

Mr Johnson, who is receiving daily intelligence briefings from security chiefs, was understood to be working with allies to provide further defensive and economic support to Kyiv, with an announcement touted for the coming days.

Downing Street issue statement on Russia Ukraine crisis

A Downing Street spokeswoman said: “The crisis on Ukraine’s border has reached a critical juncture. All the information we have suggests Russia could be planning an invasion of Ukraine at any moment. This would have disastrous consequences for both Ukraine and Russia.

“There is still a window of opportunity for de-escalation and diplomacy, and the Prime Minister will continue to work tirelessly alongside our allies to get Russia to step back from the brink.”

Foreign Office advice has been changed to urge UK nationals to immediately leave Ukraine by commercial means while they still can.

With other Nato allies issuing the same advice to their citizens, the shift represented a heightening of concerns in the West that Russia could mount an invasion.

Here is everything you need to know about the ongoing tensions.

What is the cause of tension in Russia and Ukraine?

The current difficulties date back to the overthrow in 2014 of the pro-Moscow Ukrainian government of Viktor Yanukovych, prompting fears in the Kremlin that the country was moving into the orbit of the West.

President Vladimir Putin responded by sending in troops to annex Crimea while Russian-backed separatist rebels seized territory in eastern Ukraine in bloody fighting with the Ukrainian military.

Russian President Vladimir Putin wants Nato to draw back forces from eastern Europe (Matt Cardy/PA)

Why is the West so concerned now?

For months, the US and other Nato allies have been warning of a massive Russian military build-up on the Ukrainian border, triggering fears that it is preparing another incursion against its southern neighbour.

Latest estimates suggest Moscow has 130,000 troops massed in the border region and in neighbouring Belarus, close to the strength some analysts believe would be needed to mount a full-scale invasion.

Russian and Belarus forces are embarking on large-scale military exercises which, some believe, could provide cover for an attack.

Will the West intervene militarily if Russia does attack?

It is very unlikely. Ukraine is not a member of Nato and so there is no obligation on alliance members to come to its defence and there is little appetite in Western capitals for a military conflict with Moscow.

Boris Johnson in Kyiv last week for crisis talks with Ukrainian President Volodymyr Zelensky (Peter Nicholls/PA)

Some allies have been sending military support to Kyiv – Britain has despatched 2,000 anti-tank missile launchers and a small group of military trainers, but they are expected to leave over the weekend.

There have also been moves to bolster alliance forces in Nato’s eastern members – with the UK sending additional troops to Poland and Estonia.

However, the main response has been the threat of crippling economic sanctions – including measures targeting the private wealth of President Putin’s cronies in the West.

How has Moscow responded?

Mr Putin has repeatedly insisted that Russia has no intention of carrying out an invasion.

He has however issued a series of demands intended to curb the influence of the West in eastern Europe, in what Moscow traditionally regards as its sphere of influence.

They include calls for guarantees Nato will not admit any new members – including Ukraine – and the drawback of alliance forces in the region.

Boris Johnson during a meeting with Nato secretary general Jens Stoltenberg (Daniel Leal/PA)

What is the reaction to that in the West?

Nato has been adamant it will not accept limits on the nations that it admits to the alliance.

Boris Johnson has however said there is “a conversation to be had” about force dispositions in eastern Europe.

More controversially, French President Emmanuel Macron, who held talks with Mr Putin in Moscow, has raised the idea of the “Finlandisation” of Ukraine, with Kyiv becoming neutral in the way Finland was during the Cold War.

What is the advice to British citizens in Ukraine?

On Friday night, the Foreign Office said all British nationals in Ukraine should “leave now while commercial means are still available”.

British ambassador to Ukraine Melinda Simmons was remaining with a “core team” in Kyiv, but some embassy staff and their families were being withdrawn.

But UK nationals were warned by armed forces minister James Heappey that the RAF would not carry out Kabul-style airlifts for any citizens who do not leave before fighting begins.

The White House has also urged US citizens to flee Ukraine, as have Nato allies Canada, Norway and Denmark, as well as non-alliance ally New Zealand.

Will Russia and Ukraine go to war?

With an estimated 130,000 troops on Ukraine’s borders, some analysts believe the Russian build-up has gone too far for there not to be some kind of military incursion.

Ultimately, however, it comes down to President Putin and what he calculates are the potential risks and benefits as the West struggles to put on a united front.