Ukraine conflict: What is Nato and how has it responded to Russia's invasion?
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Nato is sending hundreds of extra troops to eastern Europe to guard against a possible attack on its territory from Russia.
Germany has joined other Nato member states such as Britain and is sending weapons to help Ukraine fight the Russian invasion. The EU says it will provide hundreds of millions of pounds to help arm Ukraine.
Russia has put its nuclear forces on "special alert" to counter what it calls Nato's aggression.
What is Nato?
Nato - the North Atlantic Treaty Organization - is a military alliance formed in 1949 by 12 countries, including the US, Canada, the UK and France.
Members agree to come to one another's aid in the event of an armed attack against any one member state.
Its aim was originally to counter the threat of post-war Russian expansion in Europe.
In 1955 Soviet Russia responded to Nato by creating its own military alliance of eastern European communist countries, called the Warsaw Pact.
Following the collapse of the Soviet Union in 1991, a number of former Warsaw Pact countries switched sides and became Nato members. The alliance now has 30 members.
Why is Nato not planning to intervene in Ukraine?
Ukraine is not a member of Nato, so the alliance is not obliged to come to its defence.
However, it is a "partner" country, meaning there is an understanding it will join at some point in the future.
Some Nato nations have been sending weapons to Ukraine to help its defend itself against Russia.
The US sent weapons $200m (£149m) to Ukraine, including Javelin anti-tank missiles and Stinger anti-aircraft missiles, and allowed other Nato countries to supply Ukraine with US-made weapons. It says it will now send an extra $350m (£261m) in weapons including Javelin anti-tank missiles, anti-aircraft systems and body armour.
Britain has sent 2,000 short-range anti-tank missiles and says it will send more weapons in the future.
Germany is sending 1,000 anti-tank weapons and 500 Stinger missiles. It has also dropped some restrictions on other countries sending German-made weapons to Ukraine.
The Netherlands is sending 200 anti-aircraft missiles. Belgium is sending 2,000 machine guns and fuel. Sweden is sending 5,000 anti-tank weapons, helmets, body armour and rations.
Poland has started sending ammunition, while Estonia and Latvia have begun to send fuel, anti-tank weapons and medical supplies. The Czech Republic has said it will send guns and ammunition. Slovakia has said it will send ammunition and fuel.
The EU has said it will be spending up to 450m euros (£376m) to fund the supply of weapons to Ukraine. It's the first time in its history the EU has helped provide arms for a warzone.
One of Russia's demands before the invasion was that Ukraine should never be allowed to join - something the alliance refused to agree to.
Russia fears Nato has been encroaching on its territory by taking on new members in eastern Europe and that admitting Ukraine would bring Nato forces into its backyard.
Nato says it is a purely defensive alliance. Its secretary-general, Jens Stoltenberg, condemned Russia's invasion of Ukraine as a "brutal act of war".
China joins Russia in opposing Nato expansion
What presence does Nato have in Eastern Europe?
Nato already has troops stretching from the Baltic republics in the north to Romania in the south.
They were stationed there in 2014 after Russia's annexation of Crimea and are designed to act as a "tripwire" in case of a Russian attack.
Russia's invasion of Ukraine has heightened concerns among members in eastern Europe.
Nato is now deploying elements of its 40,000-strong Response Force to Eastern European countries bordering Russia and Ukraine.
It has 100 fighter jets on high alert and has 120 ships, including three carrier groups, patrolling the seas from the far north to the eastern Mediterranean.
"We will defend every ally and every inch of Nato territory," said Mr Stoltenberg.
President Putin of Russia has put the country's nuclear forces onto "special alert" because of "aggressive statements" by the West.
The announcement does not mean Russia intends to use the weapons.
The US immediately condemned his decision calling it an "unacceptable escalation".
The US has committed to sending more troops to Europe, but President Biden said they will not be fighting in Ukraine itself.
The extra troops will join the four multinational battlegroups Nato has in Estonia, Latvia, Lithuania and Poland, and its multinational brigade in Romania.
UK troops will not fight against Russia - Wallace
How did Nato step up defences before the conflict?
In the months leading up to the conflict, the US sent nearly 3,000 extra troops to Poland and Romania to reinforce Nato's eastern borders, and put another 8,500 combat-ready troops on alert.
The UK sent 350 more troops to Poland and doubled its strength in Estonia with an extra 900 troops.
Nato has stepped up its military defences in eastern Europe
It sent more RAF jets to southern Europe and two Royal Navy warships to patrol the eastern Mediterranean alongside other Nato warships.
It ordered 1,000 troops to be in a state of readiness to provide support in the event of a humanitarian crisis caused by a Russian invasion of Ukraine.
Denmark, Spain, France and the Netherlands, also sent fighter jets and warships to eastern Europe and the eastern Mediterranean.