In 2008, NATO promised that Ukraine and Georgia could join the western military alliance. Russia invaded the latter four months later.
So, while Putin is around no country in the previous Eastern Europe is safe from Russian invasion, so everyting needs to be done to safeguard the independence of all countries.
Many of these countries are now members of NATO and many others wish to join, but NATO is there as a defensive organisation not as an aggressive one, so all Putin needs to do is leave every independent country alone and not invade anyone on the pretext that he is safeguarding Russia from NATO invasion, which will not ever occur unless Putin instructs his forces to invade first.
This he did with Crechnia, Georgia, Crimea and now East Ukraine, now none of these areas were in NATO although some wished to be, so Putin got in first or so he thought, but his last invasion could well be his undoing for Ukraine was not going to let Putin’s invasion of East Ukraine without challenging, with the help of Western arms aid, but not Western forces. This aid needs to be continued and much increased to safeguard further invasions by Putin. For if Putin suceeds in the invasion and defeats Ukraine, it will not be long before he embarks on further conquests. So, it is more than essential that the aid is continued and be greatly increased, as Putin only understands power, previously his own assumed power, but now the power of the West in supporting Ukraine with weapons.
He knows he and Russia is no match for the combined West and he will go to any lengths to undermine the West. The West have no wish to harm Russia, but while Russia is controlled by Putin they have no choice and that is why sanctions have been imposed, but they will never invade Russia, unless Putin is foolish and he starts to invade a country in the NATO alliance.
Yes, he could instigate a Nuclear Strike, but, if he does this would immediately trigger a Western Nuclear Strike against Russia, from which Russia would never recover from, so Putin would then be responsible for the annihilation of Russia, which he has no wish to be.
So, the West needs to continue supporting Ukraine with arms to push Russian troops out of all Ukraine and that will be Putin’s humiliation and his eventual downfall. But when this occurs Russia will still have to be watched for whom will be the new President of Russia, lets hope not another aggressor like Putin.
But it is not just the Russian invasion of Ukraine, for there are many war crimes being found in the formerly Russian occupied parts of East Ukraine, which have been done to Ukrainian residents by Russian forces.
So, it is not just the defeat of Russian forces in Ukraine, but the prosecution of those Russian forces responsible for commiting the war crimes which has to be dealt with, with the final responsibilty down to the Russian President Vladimir Putin.