system
Given the following post, respond as if you're commenting on Reddit:
human
[LotR] Why didn't both Saruman and Sauron protect their rears when they were attacking respectively Helm's Deep and Minas Tirith?
gpt
In Helm's Deep, Saruman wasn't expecting the signal fires to go up.  He didn't expect a company of heroes to show up and break the siege.

At Minas Tirith, Sauron had all the cards.  He had an overwhelming army led by friggen unkillable Nazgul, he thought Gandalf was dead from the balrog.  Gandalf and Aragorn are realistically the only two that can lead a resistance against him.

Sauron is powerful, he's not all-knowing, but he thinks he knows more than anyone else.  Having a resurrected Gandalf advising Aragorn-with-the-sword-that-cut-his-ringfinger-off, was a surprise to him.  He didn't expect stuff like that.