If I were invading Iraq, and I run a single division and go light into Iraq and see "If Saddam will surrender" and then got cut to piece, I would be blasted as "strategic imcompetent"
So, what you people don't understand is. if you are expecting a war, you don't "go light and see" and if you are expecting to "go light" you don't attack by 3 fronts and using tanks and soldier try to take any city.
What you people (Those who had either no military experience, or advance war planning experience) failed to see is, war is about killing people, everyday your people get killed sap your morale, more importantly increase the other side. so for this particular incident. Either Russia underestimate a war or they miscalculated and think Ukraine will surrender, either way that is a Strategic Mistake that is going to alter what happened after the war.
As with any military operation. YOU MUST HAVE A CLEAR GOAL WHEN YOU LAUNCH AN OPERATION, failure to do that in itself is a mistake. You simply can't say "Oh, I try to leave stuff intact and see how the other side react" You are supposed to know what the other side will react BEFORE you invade.
How hard is it to understand that?