There are multiple NATO countries that already border Russia. If Putin takes Ukraine it will nearly double the amount of NATO countries that will border Russia.
That is true now, but what about 23 years from now (23 years being how long it was between the defeat of Germany in WWI and the Germany's invasion of Russian lands)?
Putin thinks Russia's military strength will decline relative to the US, which is why he is widening Russia's buffer to invasion now even though it will probably be many years before it is needed.
