Complex sentence: Before the 1990s historians could only speculate about its existence and content until in 1997, during a joint Finnish-Russian project, the Russian professor Oleg Rzesevski discovered the protocol in the Moscow Kremlin.
Split into simpler sentences: Before the 1990s, historians could only speculate about its existence and content. In 1997, during a joint Finnish-Russian project, Russian professor Oleg Rzesevski discovered the protocol in the Moscow Kremlin.