This situation is the fault of the EU/NATO/The West. After the WW, these countries could have taken the opportunity to become neutral - favour neither the US nor Russia/Eastern Bloc.
Instead, NATO took the opportunity post-war to expand Eastward, whilst conducting military ops in Serbia and Libya - countries far away from the North Atlantic. NATO stands for the North Atlantic Treaty Organisation, yet they've been continuously expanding East over the last few decades, conducting ops in Middle Eastern & Eastern European countries, countries that used to belong to the Soviet Union.
Ukraine are a long way from the North Atlantic, yet the US and Ukraine are so adamant in joining forces - why wouldn't Russia see this as a threat? Russia have a second-rate economy, the only scope they have is aggression, so I don't understand why people are surprised. Putin has been very clear about his intentions - he wants to revivify the Soviet Empire at the distaste of NATO. Whilst I obviously do not agree with Putin's actions, I am part Moldovan which is a very pro-European state, I can't help but think that the West have been 'gaslighting' Russia and the 'East' (Iran, China, etc) for decades. The West underestimate Russia and these Eastern countries so much to the extent that we've been dealing with Russia, made deals with China, Iran, the Taliban, and then we're surprised when those deals aren't adhered to?
We as the West have this belief that everyone wants to live the way we do - freedom of speech, equal rights, etc - this is why we are so gullible when dealing with these Eastern countries.
Putin has been very smart when you consider all sides, Biden pulling out of Afghanistan for no reason was essentially a green-light for a full-scale invasion of Ukraine by Russia, and potentially/probably Taiwan by China. China are probably licking their lips looking at this situation - after all, they invaded Hong Kong last year and no one batted an eye.
Sanctions imply that military action is off the table. Putin knows this, but the West don't seem to understand that Putin can absorb basically any sanction you give him.
The West are weak, the US Government under Democratic leadership are weak - it's no coincidence that Putin has invaded during the last 3 Democratic offices, but not during Trump/Republican offices. Boris Johnson, Prime Minister of the UK where I live, is delusional. Justin Trudeau is tyrannical. Macron is not sane and Germany are stuck on the fence. Knowing this, if you're Russia and China, what is actually stopping you from invading surrounding territories?
You are right about the sanctions. The only thing Putin cannot absorb is bombs and bullets.
Unfortunately for Ukraine, they are left to fight alone.
There will be no boots on the ground from the West.
If I were in NATO command I would immediately plan how to disable Crimea and Belarus.
Take over Kaliningrad, blow up Nord Stream I and II. Arrest and intern all Russian citizens abroad.
If I were in the Ukrainian army command, I would order scorched-earth policy, blow up all bridges, railroads, airports, and roads.
Blow up all gas pipelines from Russia, destroy infrastructure. Prepare for a guerrilla war.