I think when (in the words of Jack Matlock, the former US ambassador to the USSR) "
[the USA] in effect, supported an illegal coup d’etat that changed the Ukrainian government in 2014, a procedure not normally considered consistent with the rule of law or democratic governance." Russia decided they couldn't be sure the existing agreements on Crimea would be adhered to. Negotiating an agreement is one thing, sticking to it is another. Russia is plainly a guilty party but Ukraine has made it clear it has no intention of honouring the Minsk II agreement, which required a new constitution for Ukraine and more autonomy for the Donetsk and Luhansk regions. Here is a 2018 report from a Ukrainian source:
https://www.unian.info/war/10391709...kraine-without-minsk-deal-breach-adviser.html "not a single provision of the Minsk deal has been implemented by 100% as of today." In the lead up to Russia's invasion the Ukrainian parliament proposed a law on "the reintegration of Crimea and the Donbas", which if passed would have breached the Minsk II agreement. These sort of things aren't sensible or helpful.
These questions over Crimea and the Donbas aren't new either. This a news agency report from August 1990:
"
Russia warned neighbouring Soviet republics on Monday that it would not let them secede from the Soviet Union taking large Russian-inhabited areas with them.
A statement issued in Russian President Boris Yeltsin's name said the Russian Federation reserved the right to review its borders with any adjacent republic which left the Union.
His spokesman, Pavel Voshchanov, who signed the statement, told reporters at the Russian parliament this referred mainly to northern Kazakhstan and to the Donbass region and the Crimea in the Ukraine."
Yeltsin eventually backed down but it's clear Russia had concerns about borders and Russian populations in Ukraine even before the dissolution of the USSR. On Crimea specifically, in the early 90s there were a series of referendums where the people of Crimea repeatedly voted against being part of an independent Ukraine. Even before the break up of the USSR Crimea had voted to secede from Ukraine. In February 1992 the autonomous Crimean parliament voted overwhelmingly (118-28) to declare independence from Ukraine. The Ukrainian government responded quite ruthlessly, with The Times reporting that they had declared the vote unconstitutional and banned a proposed confirmatory referendum on independence. A spokesman for Leonid Kravchuk, the Ukrainian President, said "
If we cannot solve this through political dialogue, the situation will resemble that of Northern Ireland in terms of the violence involved."
So this is not simply a question of Vladimir Putin suddenly finding an excuse to expand his empire. All of this has brewing for a very long time, there have been bad choices on all sides and there are very real, very difficult questions here about nationality, self-determination and language in some parts of Ukraine (if Canada treated its French speaking population the way Ukraine treats its Russian speakers there would be absolute uproar) that are not new or created out of thin air by Putin.
If the question is what should be negotiated, Minsk II has to be the starting point. Ukraine would need to concede more autonomy for its Russian-speaking regions and agree to reform of language laws in Ukraine for starters. Russia would probably also like the US military to leave Ukraine. Russia would obviously need to withdraw troops and there might have to be some attempt to address the various dreadful things that have been done in Ukraine. Crimea is likely the biggest problem because it's never been at all clear whether the people of Crimea want to be part of Ukraine. Ideally you'd have a free and independently-monitored referendum to decide the future of the region but I doubt that's practical. That can would probably get kicked down the road again.