READ: https://en.wikipedia.org/wiki/Donbas https://en.wikipedia.org/wiki/Euromaidan https://en.wikipedia.org/wiki/Viktor_Yanukovych https://en.wikipedia.org/wiki/Minsk_agreements
In 2014, Prime Minister of Ukraine, Viktor Yanukovych, led his government to reject a deal to put Ukraine on a path to EU and NATO membership.
This immediately let to the Euromaiden protests, the Ukrainian Revolution, and attacks on ethnic Russians within Ukraine. leading to the divisions internally between the Donbas region and the rest of the country.
Crimea, which was part of Ukraine but ethnically Russian, was taken by Russia at this time. Look on a map for Crimea and the Sea of Azov, and you will see why this was so easy to accomplish.
The parties within Ukraine agreed to stop fighting and the resolution was the Minsk Protocol. The Minsk Agreements were immediately violated by both sides. Both sides were brought back to the table to draft another cease-fire agreement, known as Minsk 2.
The second Minsk Accords were also immediately violated and people have been dying in that area ever since.
The real question is: Why did the UN never move to stop these killings? Why is the West focused on Russia instead of the problem that has been at their doorstep for literally eight years now.
People in Eastern Ukraine have been dying every month for eight years. The opportunity for diplomacy has been rotting on the vine for eight years.
The borders of Ukraine were created during Soviet times by the U.S.S.R., not by the people of Ukraine. The Post-Soviet countries of Yugoslavia and Czechoslovakia similarly suffered from borders that forced people who speak different languages and come from different cultures to live together.
Yugoslavia fought a bloody and costly internal war over this before separating into multiple countries. Czechoslovakia fared somewhat better before it split into multiple countries.
Ukraine is now splitting into multiple countries. The UN for eight years did nothing to enforce the Minsk Accords to stop the fighting and allow that to happen peacefully, so Russia decided enough is enough.