I don't think the land bridge is the main issue. I think Putin would have preferred this to go piece by piece, Crimea first as it has most Russian support within the populus. However, since the Eastern Ukrainians rose up too, he's supported them to avoid the long term problems from not doing so (viz. US -encouraged revolts in Iraq being abandoned). Unfortunately, they proved unable to win with minimal support, then he handed sophisticated weapons to them, but being a bunch of yahoo's they promptly shot down an airliner, and now as the level of support he's had to provide has risen to the 'bleedin' obvious' stage, he's up a gum tree. The Ukranians have steadfastly avoided giving him a legit excuse to "provide fraternal support" (i.e. invade), and the depth, cleverness and effectiveness of the economic sanctions were a surprise to him (and most of the rest of us, me included).
Stand by for plan 'B'..........