Sonnet 15. The Trinity MS has the following for a title: On ye Lord Gen. Fairfax at ye seige of Colchester

Written in the left margin at the top of the poem is:

on ye forcers of Conscience
to come in heer
turn over the leafe

Fairfax. Sir Thomas Fairfax lead the New Model Army against the Royalists at the siege of Colchester in 1648. See brief history in Encyclopædia Britannica.

Hydra heads. The Hydra was a monster that sprouted two new heads when one head was cut off. Horace envisioned civic virtue putting an end to the "hydra of rebellion" in Odes 4.4.61-2.

false North. The Scots broke The Solemn League and Covenant when they sent troops to support Charles I in 1648. See the brief history in Encyclopædia Britannica.

impe. To strengthen or improve a wing by grafting on feathers. See OED2.