There are many different ways to skin a cat.......
Using military resources to actually fight a "hot" war to secure your objectives is a tried and tested method, with very mixed results historically, albeit quantity usually prevailed over quality.....
Using military assets as a threat to pressurise others to further your objectives is another more subtle approach. It requires merely the perception that you have sufficient forces at your disposal to make any exchange more painful than your opponent is willing to bear, and this seems to me what we are seeing at present.
Putin wants to put pressure on the expansion of NATO, and to cut a long story short is now having negotiations and discussions of a nature he would otherwise not have been. He has also exposed the reality that Ukraine's allies are willing to support it, but only so far. A bit more than tea and sympathy, but forget the cavalry riding over the hill to the rescue. However decrepit Russias tanks etc might be, they would inflict far more cost on our military than we would be willing to invest to save Ukraine (is any of this sounding familiar?) . So for the investment of some diesel to move the kit to the border, a win for Vladimir.
Such territorial ambitions as he may have towards Ukraine are better served by covert means, which are likely to prove particularly effective in the Russian supporting regions of the country, and offer him the protection of deniability.
Oh, and while we are threatening economic sanctions, what about Gas?