John Milton's poem, "To the Lord General Cromwell, May 1652" is a poem of praise for the Puritan leader who ruled England for several years after the insurrection that removed Charles I of England (and the monarch) until the Restoration of the monarchy (Charles II) to the throne.
Charles I had been extravagant with the funds, indicating that he was more concerned for himself than the well-being of England. The Puritan army defeated the Royalist army (of Charles I) in the English Civil War. Cromwell, a Puritan leader, oversaw the business of running England during part of the Protectorate. It lasted for nine years (though Cromwell ruled for only five, until his death). The loss of Cromwell sent the state of England into chaos; Charles II was able to return to the throne.
Cromwell was a man of God. He was not connected to a specific religious group, and advocated religious tolerance.
John Milton was also a religious man, and his poem is one of praise and admiration. He saw the Parliamentary war (the Puritans) as "as the people’s fight for freedom." Similar to Cromwell, Milton (though religious) did not support one religious sect over another, advocating religious tolerance.
Milton's vision of religion was something he wrote about: supporting God in the purest form, or heresy, which he saw as man changing God's laws.
With these sentiments, it is easy to see how Milton would admire the work Cromwell was doing. The sonnet to Cromwell was composed in May, 1652, as the title indicates. The sonnet is also known as "Sonnet XVII : To the Lord General Cromwell."
In the first four lines of the sonnet, Milton praises Cromwell, stating that he has marched through a cloud not only of war but also of rude "detractions," guided by his faith and persistence to a place of peace and truth.
The following lines refer to the execution of Charles I. "God's trophies" refer to accomplishments and victories in the name of God.
And on the neck of crowned Fortune proud
Hast rear'd God's trophies, and his work pursu'd,
Line refers to the battle at Preston, a battle of two days, in August, 1648, where Charles I was defeated and the blood of the Scots ran.
While Darwen stream with blood of Scots imbru'd,
In line eight, the reference to Dunbar field refers to the support of Charles II by the Scots, who Cromwell's army quickly defeated:
...The Scots had acknowledged Charles II, on his father's execution. Cromwell invaded their country and defeated them, September 3, 1650.
In 1651, Cromwell achieved his greatest victory; referring to it as his "crowning glory," Milton includes reference to the laureate wreath, a symbol of the Roman empire for reward and praise for a great accomplishment.
...in Rome [laureate wreaths] were symbols of martial victory, crowning a successful commander during his triumph.
Lines 10-11 indicate that Cromwell's work is not over: "New foes arise / Threat'ning to bind our souls with secular chains..." indicating Milton's concern that some religious group (Catholics, Presbyterians or Episcopal, perhaps) intends to enforce secularism on those who would reject it (i.e., Milton, Cromwell).
The final lines refer to Milton's condemnation of the the ministers of the Committee for Propagation of the Gospel, who did not want the separation of church and state, as Milton and Cromwell did.