Act 5 of William Shakespeare's Macbeth moves swiftly as Malcolm moves against Macbeth in battle. Much of the act is set at the castle at Dunsinane, whereas other parts have Birnam wood as the setting. In Act 5, Scene 4, we find Malcolm's army near Birnam wood. Furthermore, the audience hears Malcolm tell his soldiers that they should each cut down a bough from a tree and carry that bough in front of him as they advance toward Dunsinane. The purpose of this will be to disguise how many soldiers their army has.
This strategem is significant because in Act 4 Macbeth heard a prophecy that he would remain invincible as long as the following conditions remained intact:
Macbeth shall never vanquish'd be until
Great Birnam wood to high Dunsinane hill
Shall come against him.
Macbeth, of course, thinks that Birnam wood could never come to Dunsinane. In Act 5, however, Malcolm's strategem causes Birnam wood to come to Dunsinane. Once Macbeth realizes Malcolm's strategy, he knows that he is in trouble.