- TLN 2218historical topical
“your leafy screens throw down”
Malcolm orders his troops to drop the branches they have been carrying as camouflage. This fulfills the witches' prophecy that Macbeth would be safe 'until Great Birnam wood to high Dunsinane hill shall come against him' (4.1): the advancing army, each soldier hidden behind a bough cut from Birnam Wood, made the forest appear to move. Holinshed's Chronicles, Shakespeare's source, describes soldiers ordered to carry 'boughs cut from the trees' to conceal their numbers.
historical Holinshed's Chronicles “Lead our first battle”
'Battle' here does not mean the whole engagement but a tactical unit of soldiers — what we would call a division or brigade. Malcolm assigns Siward and his son to command the leading division of the army, while Malcolm and Macduff take charge of whatever the plan requires beyond that. This sense of 'battle' as a body of troops was standard in Elizabethan military writing.
“Do we but find the tyrant's power to-night”
'Power' here means armed force or army, not personal authority. Siward is saying: if we can only bring Macbeth's forces to battle tonight, let us be beaten if we cannot fight — a soldier's blunt expression of confidence. The construction 'Do we but...' is an archaic conditional meaning 'if we only...'; modern English would say 'If we can just find his army tonight.'