Of course he has a choice. "Not having a choice" is just his own excuse for the choice he made. He could have continued trading with the EU while investing in the development of Russia, but that takes actual work and isn't glamorous. (Not to mention that much of that investment would probably be eaten by corruption.)
I think he saw a rapidly closing window of opportunity. A successful Ukraine would be a threat to his own rule over Russia, because there are a lot of ties between Russians and Ukrainians, and Russians would notice if a more EU-aligned Ukraine is more prosperous than Putin-ruled Russia, and that would undermine Putin's story of saving Russia's economy (which he did, between 2000 and 2008).
With increasingly closer ties between Ukraine and NATO, and Ukraine possibly even joining at some point, he has to take advantage of the fact that Ukraine wasn't in NATO yet. And NATO didn't respond to him taking land from Georgia in 2008, and the Crimea in 2014, so he figured they wouldn't do anything this time either. Besides, he'd invested a lot of effort into dividing the EU and NATO: funding extreme-right-wing parties all over Europe, supporting Trump, supporting Brexit, all things that seemed to make the EU and NATO weaker. But he vastly overestimated our weakness and his own strength.
That wasn't possible, and as far as I'm aware it remains impossible for now. I believe it is a prerequisite for joining NATO, that you should be in control of your internationally-recognized borders. Otherwise, at the moment of joining, the whole of NATO would instantly be at war with Russia, because NATO's supposedly a mutual defence treaty organisation.
Same goes for Georgia, as far as I can see; Georgia's internationally-recognized borders place South Ossettia inside Georgia. But South Ossettia remains occupied by Russia.
[Edit] FWIW, I don't think NATO is in shape to go to war in Eastern Europe. Western countries took advantage of the end of the Cold War and the "peace dividend" to hollow-out their armed forces and ammunition reserves. As a Brit, I'm particularly ashamed of Britain's conduct, which has involved committing huge amounts of money to two spiffy aircraft carriers, which still don't carry a full complement of F35s, and which can't operate without a fleet of destroyers and frigates, not to mention supply ships. And a new generation of strategic nuclear missile submarines. Having a stockpile of 122mm artillery shells would be massively cheaper, and a better use of money.
> I believe it is a prerequisite for joining NATO, that you should be in control of your internationally-recognized borders.
It’s not a formal prerequisite, but its probably a practical one.
> Otherwise, at the moment of joining, the whole of NATO would instantly be at war with Russia
There’s no theoretical reason the Accession Protocol for Ukraine couldn’t specify particular territories as excluded from the coverage of Article 6 until some specified future determination.
It is, and that's at least part of the reason why when Ukraine asked to join NATO, they were rejected. But NATO won't rule it out entirely, and Putin took that as a reason (a necessity, he claims) to invade Ukraine.
> Western countries took advantage of the end of the Cold War and the "peace dividend" to hollow-out their armed forces and ammunition reserves.
Even with those cuts, though, NATO has a far, far larger army than Russia. And far better. Though ammo shortages are definitely a big problem.
I think he saw a rapidly closing window of opportunity. A successful Ukraine would be a threat to his own rule over Russia, because there are a lot of ties between Russians and Ukrainians, and Russians would notice if a more EU-aligned Ukraine is more prosperous than Putin-ruled Russia, and that would undermine Putin's story of saving Russia's economy (which he did, between 2000 and 2008).
With increasingly closer ties between Ukraine and NATO, and Ukraine possibly even joining at some point, he has to take advantage of the fact that Ukraine wasn't in NATO yet. And NATO didn't respond to him taking land from Georgia in 2008, and the Crimea in 2014, so he figured they wouldn't do anything this time either. Besides, he'd invested a lot of effort into dividing the EU and NATO: funding extreme-right-wing parties all over Europe, supporting Trump, supporting Brexit, all things that seemed to make the EU and NATO weaker. But he vastly overestimated our weakness and his own strength.