Complex sentence: Cran was provoked and irritated by Greenberg's already outdated dogma which had held such sway in Canada as well as the United States, making him one of the the most renowned art critics in American history.
Split into simpler sentences: Cran was provoked and irritated by Greenberg's already outdated dogma. Greenberg had held such sway in Canada as well as the United States and was considered by some to be one of the the most renowned art critics in American history.