So historians are basing their opinion on policies/wars that Bush has put forth that no doubt will take years to play out.
In other words, their basing their opinion on future prediction.
How else could they base a future prediction?
Regardless of the wisdom of his policies, Bush has been a failure as a war leader because of his atrocious communication skills.
Lincoln and Churchill were handed disastrous military defeats (First Bull Run, Seven Days, Second Bull Run, Fredericksburg, the Battle of France) and verbally rallied their Home Fronts to final Victory.
Bush was handed military victories and verbally lost the Home Front to the point that the U.S. is one Presidential election away from unilaterally declaring defeat and turning tail to run. Before Obama self-destructed, the odds were that the war would be lost by 2009.