I don’t actually think he did so badly after all. Even with the interference of
Gandalf taken in to account. He had the riders of Rohan penned up in helms
deep. Were without relief, a siege, no mater how protracted, would
eventual starve them out. And let face it they weren’t likely to get any kind
of relief army coming to their aid, after all there allies in Gondor where tied
up with their own difficulties. If all went wrong Saruman could fall back
to his fortress of Orthanc, which I do not think the riders of Rohan, a cavalry
unit travelling with few goods and certainly no siege equipment, could take.
The only thing that he did not take in to account where the Ents, and who
could have predicted that these creatures, strait out of myth, would put in an
appearance and destroy all his
hopes.