This might have been averted had we ignored the UN and commenced the war several months earlier, not giving Saddam time to move anything. Is that your preference? Both have down-sides.
Not anticipating the strength of the insurgency - also a major mistake
Who didn't anticipate the strength of the insurgency? It's no stronger than I anticipated it would be.
The war planners - they were thinking of bringing the troop number to 30,000 two months after "mission accomplished"