From a New York Times column by Paul Krugman headlined “Putin’s Other Big Miscalculation”:

Despite the astonishingly effective fight Ukraine has put up against invading forces, most military experts seem to believe that Russia’s sheer advantage in firepower will eventually prevail….

Still, it’s clear that Vladimir Putin made a huge miscalculation. Russia began the invasion with a dash for Kyiv and Kharkiv by small, lightly armed forces, obviously expecting to achieve a quick victory as the defenders’ resistance collapsed. Instead, the initial attack was repelled, and the follow-up with tanks and artillery has been bogged down — in some cases literally, because it’s still mud season in much of Ukraine.