The witches prophesy foretells that macbeth will be safe until birnam wood comes to dunsinane which seems impossible, but comes true. how?

Answer: Macduff and his men cut off tree branches and hide behind them while approaching Macbeth's castle.

Explanation:

In Shakespeare's Macbeth, Macbeth kills King Duncan in order to become a king, and eventually comes to the throne. However, his life turns into nightmare and he lives in constant fear that someone is going to kill him out of revenge.

The three witches tell Macbeth that he would not be defeated until Birnam Wood comes to Dunsinane. This makes Macbeth relaxed, as he finds it impossible for the trees to move. However, Macduff's army cuts off the tree branches on their way to Macbeth's castle, which, if observed from the distance, looked like the trees were moving. That is how the witches' prophecy comes true.