Bush lies about which mistakes were made
I haven't heard anyone challenge George Bush's claim that we won the war in Iraq much faster than anyone expected. In fact, it took several weeks longer than Rumsfeld promised. Remember the week when everyone started complaining about "quagmire". Then the troops entered Baghdad and the entire regime crumbled and they had no plan to secure the peace.
The truth is that the Republicans "miscalculation" wasn't underestimating the speed of victory. Their fatal miscalculation was to overestimate the receptivity of the Iraqi people. It wasn't as if the US would have been any better prepared to rebuild Iraq if they'd had an extra six weeks of fighting.
The neocons promised that Iraq was ripe for democracy. Somehow, they expected that Iraqis would greet the Americans with flowers. The Americans didn't budget men or money to rebuild Iraq because they didn't think they'd need to. They hoped to occupy it, turn it over to private interests, install a friendly government, and leave. They weren't nearly prepared to stop the looting, to deal with the sectarian divisions, or even to secure the outlying areas of the country. They weren't prepared to secure the borders, leading to a huge influx of foreign fighters who are now killing our troops. They weren't even prepared for the decrepit state of Iraq's oil industry, which they had hoped would bankroll their imperialist adventures.