European Union leaders will declare the start of accession talks with Ukraine at their summit in Brussels, their chairman Charles Michel said on Thursday in a surprise announcement.
The talks themselves are likely to take years.
Ukraine, which has a population of 44 million and is geographically bigger than any EU member, presents some unique challenges for admission to the 27-member bloc:
MONEY
Ukraine's per capita GDP is less than a third of the EU average in terms of purchasing power, which means that it would immediately become a net recipient of funds to equalise the standard of living and support its huge agriculture sector.
An internal EU study in July showed that if Ukraine were a member now, it would get 96.5 billion euros ($106 billion) under the bloc's Common Agriculture Policy over seven years and another 61 billion euros under the EU's cohesion policy, which is aimed at equalising standards of living across the bloc.
The EU study said that in total, over the EU's 7-year budget, Ukraine would be eligible to get 186.3 billion euros. This would mean that many countries which are now net recipients of EU funds would become net contributors and existing net contributors would have to pay more. This would be a big problem for most of the EU's current 27 members.
AGRICULTURE
Ukraine is an agriculture powerhouse with arable land of 41 million hectares, compared to France's 30 million hectares. It already sells most of its farming products to the EU. But as an EU member it would become part of the EU single market with no tariffs or quotas, where goods can move freely across borders.
Officials expect Ukraine would probably increase its farming production and exports to the EU, possibly swamping whole sectors and markets. This would likely trigger a backlash from farmers across the EU, creating big pressure on governments.
LABOUR
EU membership would make the whole of the EU's labour market open to millions of less well paid Ukrainian workers. A massive inflow of Polish workers to Britain after Poland's EU accession in 2004 was one of the factors which led to Brexit. Other EU countries introduced lengthy transition periods before opening their labour markets to the new EU members from the east.
SECURITY
EU treaties oblige members to help "by all means in their power" another EU country that is victim of armed aggression on its territory. If Ukraine became an EU member while the war with Russia was ongoing, EU countries would have to respect that.
The EU would also acquire a long new border with Russia and Belarus, with implications for security, migration and defence.
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