The Ukrainian government did not miscalculate, it’s defending its authority against foreign-backed forces. It is the responsibility of a government to face down armed challenges to its authority and maintains rule of law lest it becomes subject to such armed challengers and risks becoming a failed state.
Putin in the Kremlin considers the maidan revolt a threat to his throne. If Ukraine succeeds and were to enter into the EU and were to show only token progress, the challenge to the way the Kremlin operates would be significant. Revolts like the one that took place in Moscow in 2011 and 2012 have shaken the political elite in Moscow. The perception that the US is staging the protests fits right in the conspiracy theory that the Kremlin advances as the bottom line excuse for its violence.
And now Putin is in a catch 22, on the one hand he doesn’t have the popular mandate to invade and even if he did there’s no ‘exit strategy’ here which is something that the Russian army needs. The vast majority of Ukrainians also in the East identify themselves as Ukrainian and would hate a return to Moscow rule, especially one imposed with force. The Russian army would be a force of occupation, which in its disguise as auxiliaries to the rebel forces it already is, and would need to be there forever in order to impose Moscow’s will while the rest of Ukraine would have veered sharply and this time wholeheartedly away towards the EU and NATO. A new dark border would run through Europe only to validate Mr. Putin’s world view. The cost to the Russians and their economy apparently is secondary. Even the oligarch robber barons who helped secure Mr. Putin’s ascendancy seem to be forced to take a back seat. The interests of Gazprom and the other giants in the energy field are not important nor that Russia’s customers can count on a secure supply of energy this winter. All this must make way for the vagaries of a paranoid Tzar.