THE comparison between the Crusades and the September 11 terrorists in a Victorian textbook was deliberately provocative and designed to spark debate, teachers said yesterday.

The idea that the Crusaders and their fight in a holy war shared similarities and ''moral equivalence'' with the September 11 terrorists was intended to teach students how to support an argument, educators said.

The book, Humanities Alive 2 developed for Year 8 students, was criticised this week by Melbourne University historian Barry Collett for being historically inaccurate and misleading in its depiction of the Crusades and the church during the Middle Ages.

Victorian president of the Australian Education Union Mary Bluett said the text relating the Crusades and September 11 was purposely provocative to spark discussion and tease out ideas from students.

''Clearly there's sensitivity around it and teachers as professionals would handle that sort of debate very carefully,'' she said.

Ms Bluett said the aim of the exercise was not to teach students that there were similarities between the Crusades and September 11, but to teach them the principles of mounting an argument.

''It's really about teaching young people to analyse the words being said, think about their response and justify their response. It's a tool for teaching them how to advance an opinion and back it up,'' she said.