I definitely agree with your second sentence. There's nothing can be done to undo it. (Short of a showdown over nuclear war...and we might lose even that.)
I agree with your first sentence if you mean that there was nothing to do once this started to rumble in the Ukraine.
However, the Ukraine goes back to Bill Clinton, the Budapest Memorandum, Britain, Russia, and (I think) Yeltsin. That was in the late 90's. Since then, some 17 years have gone by.
Geo-politically, Russia never would have tolerated any foreigners or any Ukrainian politicians threatening Russia with having no access to the Ukraine.
The only course that would have prevented this, as I see it, was for those early Ukrainians and foreign ministers to have tried to launch Ukraine as a European version of Hong Kong or Singapore, a free, innovative commerce and financial center that became a great benefit to itself but also to everyone else, including Russia.
The only reason Hong Kong went over to China was a treaty the Brits had with the Chinese from a century prior. Nonetheless, the Chinese have been hesitant to disrupt Hong Kong's commerce. (In fact, they may have copied it in many ways.)
A robust, profitable, striving Ukraine would have had the freedom that comes with being such a successful BENEFIT to Russia that Russia wouldn't have wanted to injure that.
To accomplish that, the Ukraine would have had to live with the Russian presence, but presence is not the same as control...or subjugation. Putin loses absolutely nothing by seizing Crimea or the Ukraine.
And, like you say, anyone who thinks they will dislodge without all out war is dreaming. "Oh noes...they threatened my G8 presence." whined Vladimir Putin. Is there a single soul not in the Obama Administration who thinks that's even remotely how this is going to go?
Is there anyone who thinks the Soviet military is the weak sister that was Iraq's army? Why did Iraq permit the US to stage and prepare for an entire 6 months along the Kuwait/Saudi borders? Because Iraq couldn't prevent it. Period. No sane military leader allows that to happen when he can prevent it.
Does anyone think Putin will allow us to prepare an assault from some neighboring country? Hardly. His nukes match our nukes...I think he actually has more. His military is high tech and capable. We try to stage any kind of operation anywhere near him and we're going to get bombarded the entire time if not overrun before we can even get ashore (think D-Day.)
The only ground attack possible would be a MacArthur Inchon type invasion with total surprise. (Not likely anymore with satellites and listening technology.) It's not likely because we won't have air supremacy....the Ruskies actually have high tech aircraft...and lots of them.
It is also not likely because we don't have the Marine divisions and Army divisions deliverable to do the job...combined we might have the Naval capacity to land 3 divisions (60-70 thousand troops), when Putin already has a reinforced brigade in Crimea and 150,000 troops doing an exercise with heavy armor on the Ukrainian border.
Assuming no hidden agenda, Obama blew this. He blew it pure and simple. He let the tensions get out of hand. He has brought the modicum of Ukrainian freedom back under the jackboot of subjugation.
The reality is, though, that that's probably just fine with Bill Ayers' prize student.
We have set the tone for appeasement. Obama is seen around the world as a coward and a fool.
What would happen if Russia decided that the Aleutian Islands were suddenly in dispute? Would we risk a war with Russia or just surrender the Islands one by one?
Russia’s influence is growing. Now that they have captured the Crimea, what is to stop them from slowly rebuilding the old Soviet Union?
The Iron Curtain is moving west. There is nothing in their way at this point. And they know it.