The English and rebel Scottish armies, under the leadership of Malcolm, meet at Birnam Wood. With military foresight, Malcolm orders each soldier to cut a branch and carry it in front of him as camouflage "to shadow the numbers of our host" — that is, to conceal the actual size of the advancing army.
Malcolm's hope "That chambers (bedrooms) will be safe" in the future recalls both the location of King Duncan's murder and the motif of sleeplessness that runs through the play. Menteth's assured response — "We doubt it nothing" — is in heavy contrast to the "saucy doubts and fears" that have shaken Macbeth since even before the killing of Duncan and which will return to haunt him in subsequent scenes.
The order to each soldier to "hew . . . down a bough" as a leafy camouflage is taken direct from Holinshed's Chronicles; the aim is not to hide the advancing army but to confuse Macbeth as to the exact number of soldiers. Although Malcolm does not know it, his trick will not only fulfil the second of the prophecies of Act IV, Scene 1, but it will also play upon exactly the equivocation that has troubled Macbeth's mind since he first remarked (in Act I, Scene 3) that "nothing is but what is not."
In both Act V, Scene 2 and here, Macbeth's command over his few remaining followers is said to be based on constraint, not loyalty. His heartlessness is thus contrasted with the genuine feelings of loyalty which, it is implied, are felt towards Malcolm. In Act IV, Scene 3, Malcolm announced that Macbeth, like a rotten fruit, was "ripe for shaking"; now, according to Siward, "The time approaches," and in a final couplet adds "Thoughts speculative their unsure hopes relate / But certain issue strokes must arbitrate . . . " (19-20). Once more, the impression is that the time for guesswork is over; certainty, and the assurance of goodness, must inevitably triumph over Macbeth's lack of it.
constrained things (13) conscripts
just . . . event (14) righteous criticism awaits the outcome