Why does Henry consider his fellow soldiers imbeciles?

Henry refers to them as imbeciles because he seriously believed that they would be defeated, and that they should retreat. The line, however, held strong and was victorious. Henry's refernce to their being imbeciles isn't really an insult because they'd proved him wrong. He was the imbecile for wanting them to back down from an important battle.

The youth cringed as if discovered in a crime. By heavens, they had won after all! The imbecile line had remained and
become victors. He could hear cheering.