Make very certain that this form of treachery shall never endanger us again. What is the main purpose of including such a line in this speech? To increase anger To reassure listeners To suggest reforms To threaten Japan

To reassure listeners.

The expression "Make very certain that this form of treachery shall never endanger us again" gives assurance to the listeners that such an incident will never be repeated or allowed in the future. The words "certain," and "never again" are strong expressions of finality - - that something must be stopped.