In 1932-1933, the Holodomor, "Extermination by hunger", occurred in Ukraine. This was not a natural famine, historians believe it was likely orchestrated by Stalin in a bid to suppress nationalist movements in Ukraine and resulted in 2.5-7.5 million people being killed (up to 10 million if later deaths due to health complications from starvation are included) while grain was being forcefully exported from Ukraine by the Soviet government. The Holodomor is recognized as a genocide by Ukraine and several other countries but Russia refuses to accept responsibility.
Here is a video on this issue
The official position of Holodomor-denial is still being held by the Russian government to this day.
Denying Ukrainian State/Civilization
Russia tries to deny the Ukrainian civilization, from their point of view Russian civilization started in Kiev and then moved to Moscow and they would argue that Kiev is really a historically Russian city, perhaps many westerners also perceive Ukraine as a historical part of Russia but this is not really accurate. To understand why this is not accurate, one has to understand that both Ukraine and Russia are decedents of a civilization called "Kievan-Rus" which had a capital at Kiev. Eventually the Kievan-Rus collapsed due to the Mongolian invasion, some people moved to a small trading outpost called Moscow and eventually started a new state there, others stayed behind and eventually got absorbed into the Polish-Lithuanian Commonwealth. The Polish-Lithuanian Commonwealth eventually collapse which lead to a period where Ukraine was absorbed by the Soviet Union. Eventually the Soviet Union also collapsed which lead to a Sovereign Ukraine.
Here is a map of Kievan-Rus on the left and the Polish-Lithuanian Commonwealth on the right.
From this one can start to understand the Ukrainian perspective that they have always been living around Kiev and that their history differs significantly from that of Russia even though both Ukraine and Russia share a common origin. In fact, one could even argue that Russia is actually a offshoot of Ukrainian civilization.
Here is a good presentation by history Professor, Timothy Snyder about this issue and how it relates to the current crisis: