I was telling a friend the story of the Chinese play The Orphan of Zhao. There's a massacre, I said, in which almost an entire clan is wiped out by a power-hungry minister. When two babies are switched, the orphan of the clan, the sole surviving Zhao, is adopted by the very minister responsible for the massacre. As the orphan comes of age, he learns that his adoptive father is the murderer of his clan.

Oh, exclaimed my friend, it's Argentina! Well, yes, it has resonances in contemporary Argentina, where children of the "disappeared" have found out that their adoptive parents were responsible for the brutal suppression of the left. But the story has resonances throughout the world: I have often thought of Cambodia; others might think of Uganda, or Rwanda. There is, of course, the recent history of China itself. One doesn't need to insert these echoes. They resonate on their own behalf.

The Orphan of Zhao, which I have been adapting for the RSC, survives in various versions, going back to what is no more than the lyrics for a set of arias whose music can only be guessed at. A play with songs, its earliest versions date back to the 13th century; the events it describes date back two millennia before that, between 800BC and 600BC. In China, it is constantly revived on the contemporary stage, and a few years back was the basis for an epic film, Sacrifice. A story of revenge, it's sometimes called the Chinese Hamlet; and it's certainly performed as often. Modern versions make their own radical adjustments to the plot and the script. Indeed, there seems to be no single text that presents the whole story from start to finish. It is a living piece of drama – continuously evolving and mutating.

When incoming RSC artistic director Gregory Doran approached me, less than a year ago, to write a version for the current Stratford season (where it is to play in tandem with Brecht's Galileo and Pushkin's Boris Godunov), what immediately struck me was the problem of style. How to create a poetic language that, without sounding like so much hokum, would convey a sense of the early feudal world the play depicts?

The solution I adopted was to start writing straight away, even in advance of a clear idea of how the plot would go. I wrote a song. Then I wrote another song, and I thought: the first act could be like a suspension bridge slung between these two songs. The third song followed soon after, and the fourth came in a dream, out of which I woke and set it straight down almost word for word as it remains in Paul Englishby's musical setting.

When you write a poem, you write partly with your head, but mostly with your solar plexus. It's all a matter of how things hit you down there. And when I had my four main songs in place (a fifth, a lullaby, was to follow), I felt also that I had my poetic language up and running. The play isn't written in verse. But it's written in a poetic style that suggests the feudal psychology of early China.

We workshopped the first part of the play at Ann Arbor in Michigan, and gave it a reading. Afterwards, I asked a sinologist in the audience whether the idiom sounded reasonably Chinese. Actually, he said, there are poems in The Book of Songs that sound just like the song you gave General Wei, the honest soldier who goes into voluntary exile from the court rather than participate in the corruption of the regime.

I was thrilled – The Book of Songs is the earliest poetic anthology in the world, and was admirably translated by Arthur Waley in 1937. But I'd never owned a copy; what must have happened was that a memory of Ezra Pound and Waley's versions of early Chinese poetry had been there in the solar plexus.

I got hold of the modern paperback of The Book of Songs, and for weeks read almost nothing else. Chinese translations, particularly of poetry, are sometimes completely baffling. Occasionally, our idea of classical Chinese idiom is really just an idea of American modernism; all that tasteful free verse we read in English translation comes from the age of imagism, of Pound and those who were influenced by him. It's a kind of imaginary China. Chinese poetry itself prefers to rhyme.

But The Book of Songs is something we can all appreciate; it gives us a world in which we can fill in the details for ourselves, just as our own border ballads give us a violent, heroic, poetic world. It's a world of armies on the move, of men pressed into the service of warlords, of chariots on the road.

I found, to my intense delight, that my simple kind of idiom, with its restricted vocabulary, and its direct style, came alive in unexpected ways. It quickens the pulse. This, I hope, is the imagined language of a fearsome, distant kingdom. And this, too, is the world of the modern massacre.