Putin's approach to aggressions is an interesting one. It appears he has recognised the limitations of his freedom of action, found and began to exploit loopholes.
An all-out conventional invasion, 1914-style, is apparently out of question to him. Russia lacks the forces to pull this off on a grand scale, at least without exposing itself too much.
His exploits appear to range up to army corps size instead (South Ossetia 2008) - with all other power being held in the back, as a political equivalent to a "fleet in being". This restricts the freedom of action of other great powers. Small powers can probably not pull off the same risky games for they lack this component - even if they could easily muster forces equivalent to the ones employed actively.
Traditional Cold War deterrence rested on the fear that a too bold move might lead to World War III, and the demise of European civilisation. There were no aggressive moves done in Europe proper after the Berlin blockade; both blocs were content with keeping their own line*. Bold moves were largely restricted to Asia, with proxies and at times small numbers of opposing great power troops fighting against each other**.
There as a fear that some bold, yet incremental, moves could be dared in Europe - and it was difficult to define when exactly such incremental offenses should lead to mobilisation or war. A British satire (a "Yes, Prime Minister!" episode, see 7:04 minutes and after) explained this better than articles or books ever did. Also remember the metaphor of boiling frogs.
Putin appears to have thought of this incremental approach when he decided to send
paramilitary troops without national insignias into the Crimea.
He did apparently also take into account that the Ukraine is not allied with any country.
Finally, the third ingredient; international law had been stretched somewhat prior to the move.
Putin did stretch his freedom of action in face of International Law proponents prior to the conflict with Georgia in 2008 by exposing 'peacekeeper' troops. Georgia proceeded to attack South Ossetia at some point and this included firing on peacekeepers. At that point Putin had a semi-plausible excuse for intervention. His intervention was not as blatant as the intervention of Kuwait 1990, for example. Him withdrawing after fait accompli avoided troubles as well.
The stretching of International Law for the invasion(s) of the Ukraine wasn't done by Putin himself. This damage was done by Western great powers which had a fit of arrogance and short-sightedly decided that rule of force suits them better than rule of law. Rule of law was supposedly a concept to be applied on other powers only.
Except that the "other powers" includes some great powers which evidently can behave arrogantly as well.
It would help if the same Western great powers reaffirmed the importance of international law in a non-hypocritical way. They could admit guilt, seek and accept a ruling about compensations and - most importantly - refrain from further violations.
This won't happen, of course. Only losing aggressors have to show regret in this world.
Another approach to close the loopholes would be to expand the collective defence systems; offer an alliance of some kind to the Ukraine. This is most unlikely as well. It would lead to further conflict and might end up being much too expensive. The Ukraine is not too big to fail, after all. Nothing in there is really crucial to the West (for historical reasons), while much in there is crucial to Russia.
Finally, one could tune up the reaction to incremental moves and effectively turn incremental moves into too big moves thereby. This appears to be the preferred approach among Western great power governments.
*: The Greek Civil War already ended in 1949, being the last major attempt to keep Western countries in line.
**: Soviet and Western fighter pilots battled in a couple proxy wars.