First off lets be reasonable. There is no way military planners can account for "every" possibility. There must be some limits of reasonability. And sometimes unreasonable things may occur.
For example, in the War of 1812 the British defeated the Colonial defenders in Washington and burned the city. The operation was well planned and succeeded. However, there was no planning to protect the victorious British army from the HUGE TORNADO (perhaps more than one) that set down while the British troops were setting fire to the city.
http://www.afweather.af.mil/news/story.asp?id=123042444
The end result was a serious defeat which probably reduced British strength sufficiently to allow the American victory at Baltimore which effectively won the war.
There is simply no planning for such things.
-----------------------------------
As for the Bay of Pigs fiasco - the CIA planned that OP, but they were unable to keep the lid on it. The Russian's and therefore the Cuban's were ready for it. Airpower might have made a difference, but the Soviet's were prepared with a response (in another part of the world) should JFK have decided to commit US airpower to that fight.
The plan was weak in that it depended on tight security while also depending on foreigners (the Cuban "freedom fighters").
----------------------------------
Finally, getting back to the meat of this topic, the Iraq war was ill conceived not because GWB had a bad plan for war but because he had a foolish plan for peace. His assumption that the majority of Iraqi's would stand up for freedom and embrace a Western-style democracy was pure stupidity.
What amazes me is GHB clearly knew what invading and occupying Iraq would mean, and thus chose not to invade in the Gulf war. Every reason he gave for not doing so has come to pass in under his son's invasion.
The war is unwinnable because the goals are unrealistic. We must change our goals or be defeated. At this point our primary goal should be to secure oil to pay for this fiasco, and to keep Iran from becoming the defacto ruler of Iraq.