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 with a large X drawn through it:

To The Lord Generall Cromwell May 1652
at ye Commtee
On the proposalls of certaine ministers,for Propagation of the Gospell.

Milton probably sent this poem to Oliver Cromwell sometime in May 1652 before he had taken a public position on the proposals for church settlement and suppression of heresies put forward by the Committee for the Propagation of the Gospel. Milton, Roger Williams, and Henry Vane (see Sonnet 17) hoped to avoid the re-establishment of a state church, and vigorously opposed any settlement that would enforce uniform worship and suppress all heterodox Christian beliefs. Milton himself entertained some unorthodox beliefs that conservatives were eager to outlaw.

The octave (first eight lines) celebrates the military and moral victories that brought Cromwell to power as the head of the army and of the Council of State. (He was soon to take on the monarchist-sounding title of Lord Protector.) The sestet encourages Cromwell to resist efforts by conservative Presbyterians and some independents ("new foes") to establish state-enforced and tithe-funded religious uniformity.

peace & truth. The phrase echoes the aim of The Solemn League and Covenant made between Cromwell and the Scots.

crowned Fortune. Cromwell called his victory against Charles II at the Battle of Worcester "a crowning mercy."

Darwen stream. Cromwell's army fought the Scots at this river near Lancashire in 1648.

Dunbarr feild. Cromwell defeated the Scots at the Battle of Dunbar on this site in 1650.

secular chaines. Petitions introduced to Parliament in 1652 proposed conformity to a state-controlled Protestant Church.

hireling wolves. The system of tithes, taxation for support of a state church, meant that clergymen were effectively hired and paid by the state. Milton considered this inconsistent with religious liberty. See John 10:13-14 and Matthew 7:15.