Introduction. As Stella Revard writes in her recent edition of Milton's Complete Shorter Poems, sonnets 15, 16, 17 and 22 were not published in any collections supervised by Milton:

The sonnets to Fairfax, Cromwell, Vane, and Cyriack Skinner were published posthumously by Edward Phillips in 1694, together with Phillips’ biography of Milton, in Letters of State. They had been withheld in 1673, probably because of the notoriety of the men whom they praised – leading figures of the Civil War and Commonwealth – and because in them Milton affirmed his commitment to the Commonwealth and the pursuit of liberty. (Revard 354)

These four sonnets were first published in Edward Phillips's "Life of Milton," prefixed to Letters of State in 1694 on pages xlv-xlviii. They also appear in the Trinity Manuscript, the copytext for this edition.

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

Barbara Lewalski dates sonnet 15 "sometime between July 8 and August 17" based on the events of the second civil war to which it refers (Life of John Milton 215). Milton praises Sir Thomas Fairfax, in a style like that of Torquato Tasso's "Heroic Sonnets," not only for his martial prowess and success, but for the part Milton expects him to play in establishing a new republican commonwealth following the resolution of civil strife. Fairfax, however, retired to his home at Appleton House following the execution of King Charles I.

Fairfax. Sir Thomas Fairfax led the New Model Army against the Royalists at the siege of Colchester in 1648.

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.

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