And did those feet in ancient time

"And did those feet in ancient time" is a short poem by William Blake. It comes in the introduction to a long poem called "Milton: a Poem" (1804). Today it is best known as the hymn Jerusalem, sung to music written by C. Hubert H. Parry in 1916, more than a century after Blake had written the poem. 
The words of the poem.
&lt;poem&gt;
And did those feet in ancient time
Walk upon England's mountains green?
And was the holy Lamb of God
On England's pleasant pastures seen?
And did the Countenance Divine
Shine forth upon our clouded hills?
And was Jerusalem builded here
Among these dark satanic mills?
Bring me my bow of burning gold!
Bring me my arrows of desire!
Bring me my spear! O clouds, unfold!
Bring me my chariot of fire!
I will not cease from mental fight,
Nor shall my sword sleep in my hand,
Till we have built Jerusalem
In England's green and pleasant land. 
&lt;/poem&gt;
What the poem is about.
There is a legend that the young Jesus went to England with his uncle, Joseph of Arimathea, to England and visited Glastonbury. The poem connects this legend to an idea in the Book of Revelation ( 3:12 and 21:2) describing a Second Coming in which Jesus builds a new Jerusalem. Some Christians thought that a new Jerusalem would be like Heaven, somewhere of love and peace. The poem asks if Jesus visited England and if Heaven was created in England. Blake lived in the time of the early Industrial Revolution when there was a lot of poverty and misery. Many people think the “dark Satanic mills” were the factories where many people worked in horrible conditions. This contrasts with the country with its beauty and clean air: “England’s green and pleasant Land”. The second half of Blake's poem says people should try to establish "Jerusalem" once more.
Parry composes the music.
Blake’s poem was not very well known during the 19th century, but in 1916 a collection of poems was published which included “Jerusalem”. Britain was fighting a terrible war (the First World War), and so the people needed some strong, patriotic music to give them courage. Robert Bridges, the Poet Laureate asked Parry to put it to music at a Fight for Right campaign meeting in London's Queen's Hall. He wanted Parry to write a tune that the audience would be able to sing easily. At first Parry wanted verse one to be sung by a solo female voice, but the version sung nowadays is the arrangement for orchestra made in 1922 by Sir Edward Elgar. When King George V heard it for the first time he said that he preferred "Jerusalem" to "God Save the King", the National Anthem. 
England (as distinct from Britain) does not have an official anthem and so the British National Anthem "God Save the Queen" is used for national occasions, for example before English international football matches or at the Last Night of the Proms. The song is also the unofficial anthem of the British Women's Institute. It is often sung as a hymn in churches, although people in some churches think it is not a hymn. It is also sung in many schools.