If you'd attended a musical show on Broadway in the 1930s and early 1940s you'd probably have enjoyed something closer to a musical revue than what we actually term musical theatre today. Shows such as Anything Goes (produced here two years ago), No, No Nanette, and Florenz Ziegfeld's song-and dance revues, as entertaining as they were, usually opened with a big dance number and then broke into individual songs sung by the stars. The storyline was not important, the characters were one dimensional and the dances were merely breaks in the singing. This all changed when Oklahoma! (previously entitled Away We Go) arrived from a lacklustre run in New Haven and Boston. Based on Lynn Riggs's play Green Grow the Lilacs, first produced in 1931 by the Theatre Guild in New York, Oklahoma! was a milestone that changed musical theatre.

The formation of the Theatre Guild in 1919 marked a new direction for New York theatre. Lawrence Langner and Theresa Helburn formed a partnership with the aim of breaking from what they saw as the usual Hollywood dross by offering professional performances, high-class plays, strong production values and a cohesive acting ensemble. By the mid-1920s the Guild had built a reputation as the foremost repertory theatre in New York with literary connections to playwrights such as George Bernard Shaw and Eugene O'Neill. But during the Depression it fell on difficult financial times and lost several major actors to Hollywood. After several disastrous seasons they began looking for an inherently American musical to help with their difficulties and returned to Green Grow the Lilacs. The context for the promotion of an American art of the people was the Roosevelt-inspired Federal Arts Project.

A curious sequence of events led to the creation of Oklahoma!

Richard Rodgers saw a revival of Green Grow the Lilacs in Westport in 1940 while he was still working with his first musical partner, Lorenz Hart. Two of their shows, Pal Joey and By Jupiter had yet to open and Hart was struggling with alcoholism which made his working habits erratic. Rodgers wanted him to work on Oklahoma! but Hart refused saying that the theme of Riggs's play was not appropriate for a musical. Rodgers turned to Oscar Hammerstein, who was eager to be involved, and so the outstanding pairing of Rodgers and Hammerstein began.

Hammerstein's Show Boat (1927, with Jerome Kern) is now recognized as the first modern American musical because of its believable plotline, full characterization, and for dealing with a major subject in the difficulty of achieving true racial blending in America. The show was a hit and gave a powerful presence to black musicmaking, but until his collaboration with Rodgers very little of importance followed. Oklahoma! opened the floodgates to a series of smash hits from Carousel (1945), Allegro (1947), South Pacific (1949 - produced here last summer), The King and I (1951), Me and Juliet (1953), Pipe Dream (1955), and Flower Drum Song (1958) to The Sound of Music (1959). However none of these influenced the development of musical theatre as much as Oklahoma!

Riggs's folk play, although much darker in tone, was a very important part of the making of Oklahoma! - Hammerstein publicly acknowledged this in a letter to the New York Times: "I should like to go on record as saying that Mr. Riggs's play is the wellspring of almost all that is good in Oklahoma!. I kept many of the lines of the original play without making any changes at all for the simple reason that they could not be improved on ... Lynn Riggs and Green Grow the Lilacs are the very soul of Oklahoma!"

Rollie Lynn Riggs was born in the Tulsa region of the Indian Territory that became Oklahoma. His father was a rancher and his mother was partly Cherokee entitling him to an allotment of land under the Dawes Act of 1887. He was brought up in the outlaw era, as a result, outlaws routinely appear in his plays as comic or heroic figures of a frontier myth that symbolize a yearning from social restraint. In Green Grow the Lilacs he strips them of all glamour and exposes them as a destructive menace waiting to pounce.

It is this interplay between dark and light, between optimism and the threat of violence, that lies at the heart of Oklahoma! This was the first daring new idea that was introduced. Rodgers's melodious score cleverly uncovers an uncannily prescient delineation of the serial killer in Jud Fry. He seems to be thesupreme embodiment of what Riggs sensed as a potential for evil that imperilled Oklahoma during his childhood.

The Indian Territory was one of the destinations for the American government's policy of Indian Removal from traditional homelands. This became known as the "Trail of Tears" as thousands of Aboriginal Americans, especially the Cherokees, died from starvation, exposure and disease on the enforced marches to Arkansas and Oklahoma. There were already many Aboriginal Americans living there as well as whites and escaped slaves. Cattlemen wanted the range open for their herds to roam; farmers wanted it fenced in to areas that could be cultivated. These diverse interests lead to unruly and often violent clashes over land rights.

This is the backdrop for Oklahoma! A huge move from the cliché of the happy singing cowboy to a love story that ends when the farmhand proves to be a psychopath who is killed by a cowboy in self defence. But it provided America with a strongly embodied sense of a central national myth. Its mixture of hopeful and playful songs partly eclipsed America's memory of a devastated landscape and misplaced people and became a vital component in building and maintaining America's resolve during the height of its involvement in World War II.

Until Oklahoma! the music for this genre was always composed before the lyrics. Rodgers and Hammerstein turned this idea on its head and for the first time the American musical had an important storyline that was carried forward by the songs themselves. The collaborators analyzed the characters and what made them tick, and produced a fully rounded musical play. They also asked movie and theatre director Rouben Mamoulian to direct. Mamoulian had previously directed Porgy and Bess for the Theatre Guild, and had been asked to direct Green Grow the Lilacs but at the time was working on another project. Earlier he had directed Summer Holiday, a movie based on O'Neill's Ah, Wilderness!. In the movie world he was known for his unique touch - blending movement, dancing, action, music, singing and décor into one seething entity. Others who played important roles in this production included Lemuel Ayers (set design), Ray Blackton (conductor), Miles White (costume designer) and Robert Russell Bennett (orchestrator); as well as Agnes de Mille.

Rodgers and Hammerstein were concerned that their audience might find cowboys and cowgirls verbalizing their emotions difficult to believe, so they decided to use dance as an integral element in the story-telling. They chose Agnes de Mille to be the choreographer. She utilized trained modern dancers in place of the standard chorus girls. The resulting personality rich ensemble added to the show's innovative look and feel.

Dance was not a new element in musical theatre. What was new about Oklahoma! was the combination of music and dance to further the plotline and help the audience fully understand an individual character's feelings and emotions at that point in the drama.

These and many other innovative choices - and they involved every facet of theatre from lighting to staging - made the production a hard sell. When it opened for previews in Boston, Variety magazine gave it a poor review: "No gags, no girls, no chance." Many investors sold their shares but many realized the potential of this unusual musical. Rodgers and Hammerstein made extensive revisions, including changing a minor dance melody into a de Mille set choral piece with the chorus coming down to the footlights in a v-formation singing "O-K-L-A-H-O-M-A". It left the audience cheering and gave the show its title. Oklahoma! opened in New York on March 31, 1943 to a partially-filled theatre. With no stars they could not even paper the house. But by closing the results were astounding. It set a new long-running record for Broadway musicals, with 2,212 performances; won a Pulitzer Prize for drama in 1944; ran for three years in London's West End; a second company toured the show in the United States for almost 10 years nonstop; and gave the backers a 2,500% return on their investments. Oklahoma! is still produced more than 600 times a year worldwide.