Putin invaded Ukraine because, as he has made clear, he considers Ukraine to be part of Russia and does not recognise the sovereignty of Ukraine.
He says that if there are military fighters in Ukraine from other countries he will take that as an invasion of Russia.
If ceasefire and sanctions fail, Europe has to consider that if they DO NOT put their military boots to help Ukraine, will Europe and US (no use considering what Trump thinks) be seen to accept Putin's claim that Ukraine is Russian territory?
How can we not send military to Ukraine? If we don't we will be seen to have given Ukraine to Russia, as Putin claims.
If we do, will Putin go to war against Europe? Should we call his bluff? From what Lavrov said recently, they are prepared for WW3 anyway. They want it to happen.
This week Putin said the 'special military action' is connected to WW2, which can only mean Putin is hinting that he wants to recover the USSR, He wants war. He wants to see what happens in his lifetime, wants to try out new weapons - in his lifetime.
Kim Ung wants that too. I don't think Xi Jinping wants that, but China shares a long border with Russia.
EU and NATO members will not put Putin off war. What harm can they do to him once it starts?