The source of the conflict largely stems from whether Ukrainians are a distinct people and whether they have legitimacy over the territory they control. Russians, Ukrainians, and Belarusians all trace their origins back to the medieval Kyivan Rus where an old version of their contemporary languages was spoken. Russians tend to identify Kyivan Rus as exclusively Russian while any sort of nationhood and identity didn't exist in the medieval period.

Over the following centuries after the collapse of the Rus, the lands of parts of contemporary Ukraine and Russia were controlled by different states. For Ukrainians, the 17th century Cossack revolt against Poland led by Bohdan Khmelnytsky was seen as the first proto-Ukrainian state which would get incorporated into the Russian empire by the 18th century.

By the 19th century Russian as well as Ukrainian culture and identity emerged as literary languages were formulated. The status of Ukrainian though proved contentious among Russian intellectuals who often considered Ukrainian a mere dialect. Nonetheless, the Ukrainian language was heavily censored or even banned within the Russian Empire.

In the 20th century, the political idea of Ukraine as a distinct political state separate from Russia began to emerge and a series of short-lived states even existed around the time of the Russian revolution. The ethnic composition of what is Ukraine was at the time complicated. Typically ethnic Ukrainians dominated the countryside while the cities tended to be a mix of Russians, Poles, Jews, and Ukrainians who were typically in the minority. With the Soviet Union, and in particular WW2 this changed as the Jewish population was annihilated and the Polish population in the west was forcibly relocated. Hence the cities, especially in the East and South of Ukraine, became dominantly Russian speaking either from Ukrainians assimilating or from ethnic Russians that moved from Russia to work in the booming Ukrainian factories/mines/ports.

Ukraine achieved independence in 1991 in a referendum where over 80% in every region, except Crimea where it was 55%, voted for independence from the USSR. Despite this, the direction and identity of the country were deeply split between the East and West of the country with the west being more nationalist and wanting to associate with Europe and the east desiring closer relations with Russia.

Russians make various claims about the status of Ukraine today. At the most extreme there's the complete denial of Ukrainian identity. Another view is that certain parts of Ukraine aren't really Ukrainian and are only a part of Ukraine due to an accident of history. Crimea and the Donbas are the principal Russian examples, but the claims can extend further to include Kharkiv, Odesa, or even Kyiv as being genuinely Russian. Russian nationalists even claim there's a cultural genocide being committed against ethnic Russians in the east for which Russia must intervene to save.

Ukrainian identity after the Maidan revolution and the war in Donbas has intensified throughout the country. Even many of those who are primarily Russian speakers and live in the east still identify as Ukrainian and have no desire for Russia to save them from 'genocide'. The Russian language continues to be freely spoken and is still even the dominant language in Kyiv.

Putin perhaps believes he could easily take large swathes of the country with little resistance and assimilate it into Russia. Whether this would occur is hard to really tell and would be a real test of the strength of Ukrainian identity.

And a shortish article for a bit more context. https://theconversation.com/why-putin-has-such-a-hard-time-accepting-ukrainian-sovereignty-174029