The EU and Russia are engaged in a contest to attract the countries clustered on the eastern margins of the European continent into their respective spheres of influence and control. They use some similar techniques to attract countries, though the West would argue that Russia also uses force in some cases and localised areas. They offer a customs union, some discounted or favourable access to things countries need, possible membership of an associated defence grouping, and some mutual support. The west offers EU membership of a customs union and trading system through an Association Agreement, which binds a country into a considerable volume of EU law. Russia offers membership of the Eurasian Economic Union for trade and economic collaboration. For defence the West offers to some membership of NATO, whilst Russia proposes her own Collective Security Organisation.
These competing offers or pressures can prove difficult for the buffer states caught between them. They did so in Ukraine. In 2014 protesters took to the streets in pro Western West Ukraine to topple the elected President who was trying to keep Ukraine neutral between the EU and Russia. The pro EU forces thought him too sympathetic to Russia and disliked his refusal to sign an Association Agreement with the EU. He had also been trying to ensure Russian continued use of warm water ports in Crimea which the Russian navy needs especially in winter. The decision of Ukraine to change Presidents and draw closer to the EU was seen as a reversal by Russia and led to their annexation of Crimea, claimed as part of Russia with a population said to be strongly in favour of joining Russia. Russia assumed and resented EU involvement on the side of the opposition forces to the outgoing President. Russia also then released a transcript of a claimed conversation by Victoria Nuland, the US Assistant secretary of State, to show the USA was also involved in seeking a new anti Russian government in Kiev. The USA never confirmed the tape was accurate but they did not rebut it either. It mentioned Vice President Biden as offering support for the actions, without mentioning President Obama. Clearly the Russians harbour a grudge against Biden and Nuland over Ukraine, and want to give them a tough time by providing a show of strength on the Ukrainian eastern border. Nuland is currently nominated to a very senior role in the State department.
There are similar issues of influence and loyalties in Moldova, Armenia, Kazakhstan and Georgia. President Biden and his EU allies have to work out how to avoid further attempted annexations of territory and how far they can go in allowing or containing the spread of Russian influence by other means. President Trump did let Russia gain influence in Syria as he sought to avoid war in that troubled part of the world.