The general informed his troops, "We must be ready as the enemy is preparing to attack us tonight."
I have reported the above speech as follows. Is it necessary to change 'must' to 'had'. I think it is not necessary. Am I right?
The general informed his troops that they had to be ready as the enemy was preparing to attack them that night.
You are right. If you are writing in an examination, I'd recommend that you use 'had to', especially if the examination is to be marked by non-native speakers (some of whom insist on rules they have read being followed), but many native speakers would use 'must'.