Ironically, Putin could have rolled the Russian army all the way to Kyiv in 2014-15 because his stooge, Yanukovych, had made the Ukrainian army a corrupt shell. He chose not to and now is only able to mass murder civilians and destroy Ukraine with bombs.
I think he was surprised that Ukrainian security would not back Yanukovych when Yanukovych ordered them to use force against the Maidan protesters a la Tiananmen. That made Yanukovych unable to find a safe haven from which to call in “fraternal assistance” from Russia.