Prince Rupert's Royalists attacked Parliamentarian troops at Brentford west of London on 12 November 1642. The Royalists captured the town and prisoners, but allegations of plunder alarmed London's population. Parliament rapidly assembled a much larger defensive force on the capital's outskirts.

Parliamentarian propaganda used the reported sack of Brentford to mobilise London against the king. Ballads and news pamphlets transformed a local engagement into a warning about what Royalist victory might mean for civilians.