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Ukraine is set to become an official candidate for European Union (EU) membership, the first step to joining the 27-country bloc.
EU leaders are expected to back the move at a summit being held on 23 and 24 June.
And the Ukrainian president has warned Russia may increase its military attacks in retaliation to the decision.
The European Union is an economic and political union between 27 European countries, which was created in the aftermath of the Second World War.
Goods, services and money move freely between member states. EU citizens can live and work anywhere within the bloc.
It operates a single European currency, the euro, which is used by more than 340 million people in 19 of the EU's 27 countries.
Common standards apply across member states in a number of areas including food safety, farming and employment rights.
The EU also gives grants to Europe's poorer regions to help strengthen their economies.
President Volodymyr Zelensky began the application for EU membership five days after Russia invaded his country in February 2022.
He asked to be able to join immediately, but the process could take years.
Image source, Gertty Images
There would be financial benefits to joining the EU. But Ukraine's main motive is not economic, argues Dr Zach Paikin, from the Centre of European Policy Studies think tank in Brussels.
"EU membership would establish Ukraine firmly as an independent, sovereign European state," he says. "Not merely part of the Russian world."
Firstly, the European Commission, the EU's civil service, assesses whether a country is a suitable candidate.
It considers whether it has a stable, democratic government, respects human rights, and has a free market economy.
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All existing EU members have to agree before a new country can become a candidate.
If successful, the country has to adopt all the EU's laws and regulations. This normally takes several years.
Finally, the country signs an accession treaty which all the other EU countries have to ratify.
For recent joiners Bulgaria, Romania and Croatia, the entire process took between 10 and 12 years.
Albania, North Macedonia, Montenegro and Serbia have been official candidates for years, but their applications have stalled.
Turkey became a candidate in 1999. However, its accession talks were halted over concerns about the country's human rights record.
Ukraine's neighbour, Moldova, was accepted as a candidate on the same day as Ukraine. Georgia also applied, but was told it must make further reforms.
- Largest land mass of any country in the EU: 603,550 sq km (233,032 sq miles)
- Fifth-largest population: 44.13 million
- Ukraine's annual GDP per capita is $3,724 (£3,040) – one-ninth of the EU average
- It already sells the EU 30% of all the grain it imports
Since 2017, Ukraine has been able to trade with EU states without paying tariffs, under a so-called association agreement.
The country had already paid cheaper tariffs after joining the EU's deep and comprehensive free trade area in 2016.
As part of its preparation to apply for membership, Ukraine has already changed many of its laws and regulations to meet EU standards.
European Commission president Ursula von der Leyen says "good work has been done", but that Ukraine must make further "important reforms".
Image source, EPA
These include strengthening the rule of law, improving human rights, reducing the power of oligarchs, and tackling corruption.
"There's also a need for Ukraine to build a fully-fledged market economy," says researcher Zach Paikin, "which is difficult for a post-Soviet republic."
Another challenge will be cleaning up Ukraine's court system, which has been widely criticised.
Since joining 15 years ago, Romania's national income has tripled and Bulgaria's has doubled.
The EU has given tens of billions of euros to both countries through European Structural and Investment Funds. These support economic development by, for example, building new roads and ports.
Between 2014 and 2020, Bulgaria received 11.2bn euros and Romania 35bn euros.
However, Transparency International argues a lot of this money has been lost through corruption.
Bulgaria remains bottom of the EU league in terms of wages, health and education, but Romania has risen from second poorest to sixth poorest.
Some 1.5 million Bulgarians and 4 million Romanians have used the EU's "freedom of movement" policy to travel abroad, according to the World Bank. Critics fear this "brain drain" could damage both countries' future prospects.
Although President Vladimir Putin has been very critical of Ukraine's long-standing desire to join the Nato military alliance, he insists he has "nothing against" Ukraine becoming an EU member.
However, his spokesperson, Dmitry Peskov, says the Kremlin will give the application "increased attention" because the EU is considering forming its own defence force.
"The military, defence and security components are being discussed," said Mr Peskov. "We are, of course, observing it all thoroughly."
Ukraine's President Zelensky has warned that Russia might retaliate against the move.
"Obviously, we should expect greater hostile activity from Russia," he said. "And not only against Ukraine, but also against other European countries."