Putin invaded in 2014.
https://en.wikipedia.org/wiki/Russo-Ukrainian_War
In 2014 the Ukrainians foolishly threatened not renewing the lease on Putin’s military port on the Black Sea. Something he pretty much can’t live without. Add that to the fact that most people in the Crimea wanted to break from the Ukraine so he annexed it.
Putin has not invaded and occupied the Ukraine and likely won’t try because he has the same GDP as Texas and cannot afford it. He would have a terrible time holding it as the Ukrainians hate the Russians guts. And the EU is just itching to bet it on with Putin and try to drag the US into it. No, Putin is not going to get dragged into that.