US president Joe Biden is anticipated to have the most difficult phone call of his presidency so far when he speaks to Russian President Vladimir Putin at 3pm UK time on Tuesday.

Mr Biden will warn that if Russia invades Ukraine as is feared, he will bring down the might of US sanctions on Mr Putin’s country in a way that will exact “a very real cost” on its economy.

But how has it got to this point, and what has happened to sound the alarm?

What is happening on Ukraine’s borders?

US intelligence officials have studied the movements of troops and military equipment and determined that Russia has amassed 70,000 troops near the Ukraine border and is planning to deploy another 100,000 along the border as part of preparations for a possible invasion early next year.

Some 90,000 troops were stationed in the region for massive war games in the autumn and the Ukrainian Defence Ministry says units of the Russian 41st army have remained near Yelnya, a town about 260km (160 miles) north of the Ukrainian border.

The size and location of the forces has led Ukraine to accuse Russia of preparing for a possible large scale military offensive, which would result in open war between the two neighbours.

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Ukrainian says, meanwhile, tanks and snipers are being sent to the line of contact in war-torn eastern Ukraine to “provoke return fire”, potentially providing a pretext for an invasion in retaliation.

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Why is the US so anxious?

Biden was vice president in 2014 and has keen memories of when Russian troops marched into the Black Sea peninsula of Crimea and annexed the territory from Ukraine, forcing a crisis in relations.

Russia then threw its weight behind a separatist insurgency in the east of Ukraine, which continues to this day at a low level.

A 2015 peace deal was aimed at ending the war and stopped large scale confrontation between Ukraine and rebels. But Russia now accuses Ukraine of breaching the peace agreement, and the West of not enforcing it.

Moscow has also remained concerned that Ukraine may join the NATO group – where the binding principle is an attack on any member, is an attack on the organisation as a whole. Russia wants the West to rule out Ukrainian membership.

But Mr Putin is said by analysts to resent the break-up of the Soviet Union and harbour a view that Ukraine should be part of the Russian Federation.

A recent article by the Carnegie Endowment for International Peace said Mr Putin published a treatise in July which asserted that “Ukraine is not and has never been an independent state” and “amounted to no less than a historical, political, and security predicate for invading it”.

US President Joe Biden, meanwhile, is a supporter of a fully independent Ukraine and has pledged his “unwavering support” to Ukrainian President Volodymyr Zelenskiy.

Has anything else happened to raise tensions?

According to US officials, the state of relations between the West and Russia at the moment are now permanently at the lows of the Cold War.

Added to that, a number of incidents and ongoing issues are keeping relations at their coldest since the Berlin Wall came down. They include:

  • A Russian missile test strike on a defunct satellite that forced astronauts on the International Space Station to take shelter from a cloud of debris has sparked fears about a new proliferation of arms
  • The attack on and subsequent jailing of opposition politician Alexei Navalny has left Russia largely without a functioning opposition
  • A series of cyberattacks and hacking campaigns that have targeted Americans have been blamed on Russians – including some with connections to the state
  • The ejection of diplomats for various diplomatic rows has left the US struggling to fill its embassies and operate effectively in Russia
  • Russia’s involvement in Syria’s war and supply of gas during a worldwide energy crisis has left Mr Putin with greater influence around the world

Will Russia invade Ukraine?

Some interpret the troop build-up as a demonstration by Mr Putin that Russia is determined to show NATO it needs to respect Moscow’s red lines and stop sending troops and weapons to Ukraine.

The Kremlin, meanwhile, has insisted it has no such intention and has accused Ukraine and its Western backers of making the claims to cover up their own allegedly aggressive designs. It has also said deployment of its forces on its own territory shouldn’t concern anyone.

US officials have said Moscow’s intentions are unclear, but point to Russia’s past behaviour as a reason for concern.

What might happen if Russia invades?

No decision has been announced yet but the US and Europe would have a number of options. They are reported to include:

  • Sanctions targeting Putin’s inner circle
  • Sanctions against Russia’s biggest banks
  • Going after Russia’s ability to convert rubles into dollars and other currencies
  • Disconnecting Russia from the SWIFT international payment system used by banks around the world
  • Sanctions targeting the Russian Direct Investment Fund
  • Sanctions on the Nord Stream 2 pipeline, due to bring gas into Europe from Russia

Will there be knock-on consequences?

If war breaks out, it could have an impact on the markets or the price of energy, as Russia currently supplies significant amounts of gas to several EU countries.

It will also increase pressure to protect other countries that border Russia, such as the Baltic states, where the UK already has a sizeable troop deployment.