There are many reasons why Russia has failed to conquer Ukraine, but behind them all sits a grave miscalculation: the Putin administration thought the global response would be limited because Moscow would achieve a rapid and decisive military victory. In this it was clearly quite wrong.
Russian air and ground forces have always relied upon mass over capability. While manpower and equipment depth are nice to have, the more advanced the latter, the less useful the former. The key to this is the sophistication and capability offered by platforms, systems and weapons. Ukraine has also underlined the need for a