Ukraine conflict: What is Nato and how has it responded to Russia's invasion?
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Russia's invasion of Ukraine is one of the biggest challenges Nato has faced in its 73-year history.
The western alliance is sending extra troops to Eastern Europe, but has no plans to get involved in the current conflict.
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.
The former Soviet republic has wanted to join for several years.
However, 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.
The US has also 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 US also sent weapons worth $200m to Ukraine, including Javelin anti-tank missiles and Stinger anti-aircraft missiles, and allowed other Nato countries to supply Ukraine with US-made weapons.
The UK sent 350 more troops to Poland and doubled its strength in Estonia with an extra 900 troops. It also supplied Ukraine with 2,000 short-range anti-tank missiles.
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.