This is one of the shortest scenes in the entire play. It has about ten lines. The primary purpose of this scene is to fulfill the prophecy of the apparitions that Macbeth will not be defeated until the Birnam woods move towards Dunsinane. It allows the audience to have a picture of the advancing forces. At this point, the audience is aware that it is only a matter of time before Macbeth’s reign collapses altogether.
The scene also reveals Macolm’s army’s tactics and order. He has planned the attack so that his worthy uncle, Siward, leads initial forces. After that, Macduff and Malcolm guide the troops that follow. Malcolm says that this is done “according to our order.” The expression “harbingers of blood and death” refer to the trumpets that lead the conflict.