Ukraine conflict: What is Nato and how has it responded to Russia's invasion?
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Russia's invasion of Ukraine is presenting Nato with one of the biggest challenges it has faced in its 73-year history.
The western alliance is sending extra troops to the Russian and Ukrainian borders 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.
What is Nato and how is it responding to the Ukraine crisis?
China joins Russia in opposing Nato expansion
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, but this was met by fierce opposition from the Russian leader Vladimir Putin, who said recently that Ukraine is really part of Russia.
Indeed, one of Russia's grievances before the invasion was that Ukraine should never be allowed to join Nato - something the alliance refused to agree to.
Nato's secretary-general, Jens Stoltenberg, has condemned the invasion as a "brutal act of war".
In response to concerns from members in eastern Europe, Nato has already put hundreds of warplanes and ships on alert and will be increasing troop numbers in the region.
The US has also committed to sending more troops to the region, but President Biden has made clear that they will be defending existing Nato territory, not fighting in Ukraine itself.
What presence does Nato have on its eastern European border?
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.
It has naval vessels patrolling the eastern Mediterranean and aircraft "air policing" along its borders with Russia.
Nato has four multinational battalion-size battlegroups in Estonia, Latvia, Lithuania and Poland, and a multinational brigade in Romania.
France has also offered to be the lead nation of a future Nato mission in Romania, which could involve about 1,000 troops from various countries.
Nato could send anything up to 40,000 troops from the Nato Response Force, setting up extra battlegroups in Romania, Bulgaria, Hungary and Slovakia.
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 a Royal Navy vessel 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.