Five years ago, the small nonprofit theater company Ars Nova commissioned an up-and-coming composer to write his wacky dream project, a musical adaptation of one dramatic section of “War and Peace.”

On Tuesday night, that musical, “Natasha, Pierre & the Great Comet of 1812,” now a $14 million show starring the best-selling recording artist Josh Groban, had its first preview performance at the Imperial Theater — a major moment for Ars Nova, which has never before seen a project it birthed transfer to Broadway.

But the leadership of Ars Nova was not allowed to be there.

In a stunning and abrupt severing of an unusually close partnership, the nonprofit and the show’s commercial producers, closely interconnected by longstanding personal and financial relationships, are suddenly in a bitter battle over how the nonprofit’s role in the show’s development is credited in the show’s program.

Infuriated by the dispute, the show’s commercial producers have barred Ars Nova officials from the theater and threatened to prevent cast members from performing at the nonprofit’s big annual fund-raiser by scheduling the cast album recording on the same day, according to Ars Nova.