My understanding is that the Ukrainian political system was reasonably closely balanced between west leaning and east leaning factions, so much so that west leaning figures like Tymoshenko could achieve power on occasion.

I know the Crimea is relatively small, but could its defection be enough to tilt the balance of power in the remaining Ukraine marginally but decisively in the west leaning factions favour?

If so Russia could have gained Crimea at the cost of a near controlling influence in Ukraine. The fundamental economics of pipelines and interdependency don't change, but the EU's influence in Ukraine will grow - for good or ill.

The danger is that Russia will be provoked into further annexations in eastern Ukraine "in order to protect ethnic Russians..."  This could provoke a military response by Ukraine in a bid to protect its territorial integrity and all hell could break loose, include pogroms, ethnic cleansing, mass migrations of civilians, and a hot or cold war around some de facto border in Ukraine.

Will the west really come to Ukraine's aid in this scenario, beyond the usual guff about sanctions etc.? Where will the new Russia/West border really lie before the west intervenes militarily?

Both Russia and "the West" have a vested interest in avoiding such an outcome. The question is whether cool heads will ultimately prevail and further provocations are avoided.

My best guess is that Ukraine will lose Crimea but ultimately Russia will lose a controlling influence in Ukraine. Both sides had better get used to that new reality if disaster is to be avoided.

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