That is the wrong question. The question is:
Is there some reason Europeans should go to war when Russia invades a European country in order to annex it by force?
The answer is yes. The reason is not letting force impose over things like democracy or rights. If Europeans do we(I am European) will become slaves.
It happened multiple times: The Ottoman Empire, Napoleon or Hitler and Stalin.
You don't have to explain that to a Polish person: They lived the occupation of Russia and Germany and all of them have family members that were exterminated by the germans first and then by the communists. Let alone they were subjugated over decades making then a puppet state of Russia making Russia richer and Poland poorer.
If Europeans do not oppose the dictator Putin controlling Russia, next time we will have to fight against Russia, Ukraine(occupied by Russia) and Kazakhstan, Turkmenistan and all the new conquests(of the new Russian Empire) in the same way if you do not oppose the ottomans taking Constantinople, you will have to oppose them in Vienna, when they have become much stronger.