"We have seen 5 waves of NATO expansion. Now NATO is in Romania and Poland and they are deploying their missile-attack systems there. That's what we are talking about. You need to understand, we are not threatening anyone. Russia did not come to the US borders or the UK borders. No. You came to our borders and now you are saying, 'Ukraine will join NATO and will deploy their systems there. They will deploy their military bases and their attack-systems.' We are concerned about our security. Do you understand what that means?" Vladimir Putin, press conference
Question– Is there a justification for Russia’s invasion of Ukraine?
Answer– Yes, there is. Russia was being threatened by developments in Ukraine, so it told Ukraine to either stop what it was doing or suffer the consequences. Ukraine chose to ignore those warnings, so Russia invaded. That is basically what happened.
Question– But how does that justify the invasion, after all, Ukraine is a sovereign country and sovereign countries should be able to do whatever they want to on their own territory, right?
Answer– No, that’s wrong. Ukraine does not have the right to do whatever it wants on its own territory. Ukraine and more than 50 other countries signed treaties (“at the OSCE summits in Istanbul in 1999 and in Astana in 2010”) agreeing that they would not strengthen their own security at the expense of other’s security. This is called the “indivisibility of security”, but in practical terms it just means that you can’t put artillery pieces and tanks on your driveway and point them at my house. Because that would undermine my security. Do you understand? The same rule applies to nations.
If we accept your reasoning on the matter, then we’d have to conclude that John Kennedy had no right to challenge Fidel Castro for putting nuclear weapons in Cuba. But he did have the right because Castro’s action put the US at risk of a nuclear attack. In other words, Castro had no right to improve his own security at the expense of the United States. This is no different. Putin has every right to defend the safety and security of the Russian people, in fact, that is what people expect of their leaders.