The EU was busily influencing Ukrainian opinion and politics , encouraging a pro Western line working towards EU membership. The 2014 revolution and change of Ukrainian leadership forced out a more pro Russian President and brought in a pro EU President. Putin seized the Crimea , claiming it was a very pro Russian region that should never have left Russia.
This left the south eastern provinces where there was also a larger pro Russian population. Civil war broke out with Russians encouraging secessionists. The pattern of behaviour was similar to Russia’s action in Georgia to detach South Ossetia and Abkhazia . In Moldova Russia seeks to control Transnistria. Belorus is governed by allies of Putin. To the west this is a war of Russian expansion, as Putin seeks to reunite old parts of the USSR. He claims he is supporting and assisting populations who wish to be independent of Georgia or Ukraine or Moldova as they look towards the EU but uses force to back up his claims. The West sees Putin as an aggressor seeking to gain territory by violent conquest. Whilst they wish to stop him they have understandably not wanted to go to war themselves with Russia. Ukraine has been fighting the war whilst trying to get more help from the US and EU.
To Russia the Russian speaking areas of Ukraine should belong to Moscow. They claim there are people in those parts of Ukraine especially in Crimea that want to be governed by Russia. They see it as a war against EU expansion with the EU offering membership to Ukraine, Moldova, Georgia, to push its borders closer to Russia. Russia does not distinguish between expanding borders by military action and expanding them by government decisions in the way the West does.
It is most important if there is a peace Treaty or ceasefire in Ukraine with the current front lines as a new border that the UK leave the task of policing that border to the EU.It becomes an EU border as soon as Ukraine membership of the Union is confirmed.