Link. This matter was brought up in 2014 when Russia occupied the Crimean peninsula, per the link Timmer provided:
2014 Russian Annexation of Crimea
Following months of political unrest and the abrupt departure of Ukrainian President Viktor Yanukovych, Russian troops entered the Crimean peninsula of Ukraine in March 2014. On March 18, over the protests of the acting government in Kiev, the UN Security Council, and Western governments, Russia declared the annexation of Crimea. The United States, the United Kingdom, and Ukraine called the action a blatant violation of the security assurances in the 1994 Budapest Memorandum. However, according to the Russian Foreign Ministry, “the security assurances were given to the legitimate government of Ukraine but not to the forces that came to power following the coup d'etat.”
With a tepid reaction to the annexation of Crimea, Putin and the Duma and Russia's military likely anticipated little resistance to the current incursion. Not so. Now what?
The item on the arms control negotiation and arrangement ends with a timeline.
The history of the Yanukovych ouster is, itself, separate story, as is the question of assurances given or not to Russia upon the dismemberment of the Soviet Union into separate states - assurances that NATO would not expand eastward from its boundaries at the time of the Soviet Union's end.
Whether any party has clean hands in things involves finger pointing back and forth, but the European Union is inhospitable toward a new land war in Europe. History does count for something in that respect. European land wars define much of European history, culminating in the Twentieth Century's development of nuclear weapons presenting a perspective of destruction beyond the worse in history.
Peace needs to be kept, and that explains all the European arms being given Ukraine to resist Russia. Economics are a parallel dimension, but who gets whatever natural gas profits the Russians may have forfeited is a secondary issue.
With the notion of shipping in arms, to attain peace being quaint, it nonetheless is the step which can quell the unjustified aggression. Yes, NATO was put on the border of Russia, the Baltic States, and Ukraine was tampered with by Russia, the EU, and the U.S. of A. But only Russia has sent in an army aimed at whatever conquest they first intended, with whatever resolution finally happens.
__________UPDATE________
In retrospective thinking, Timmer's post needs more emphasis. Timmer gets into the question of Russia claiming a genocide against its friends as underlying cause for the invasion, not it being a simple land grab where stern opposition was unforeseen.
Timmer emphasizes that international law offers a testing avenue for claims of genocide, with Russia declining to participate, that way.
FURTHER: Without linking to any particular online source item, it has been suggested Putin might have envisioned a popularity boost in Russian home politics, from a quick military action and victory. That kind of motivation, if actually at play, was most effectively parodied and mocked, by this video excerpt from a film made decades ago.