J. Michael Straczynski, who rebooted Marvel’s Thor in September of 2007 and turned it into a Top 10 comic, is leaving the book in September 2009 when Marvel will publish his Asgardian swansong, Thor: Defining Moments Giant Size #1. 

In an interview with Comic Book Resources, JMS explained why he decided to end his Eisner-nominated run on Thor: “The one concern at the back of my head was that of being pulled into a Big Event that could affect the forward momentum of the book and alter its direction. I've said elsewhere that in many cases -- and this isn't just Marvel, the trend is pandemic -- such an event can sometimes result in the individual books serving the event, rather than the other way around, and you have to spend months and issues afterward stitching everything back together.”

The catalyst for JMS’ decision to quit Thor was what he called “The Siege of Asgard Big Crossover Event.” After mulling over his decision, Straczynski informed Marvel Publisher Dan Buckley: “I really, really hate this, because creatively I'm having the time of my life, but I think I need to let go of the book or risk screwing it up when we hit the Event."

Despite his disappointment that he will no longer be able to “do that sort of faux-Shakespearean style of dialogue that I love so much,” JMS maintains that he wasn’t ill used by Marvel: “There were no creative differences, no animosity, no rancor... just the question of how to handle being caught in the switches by my own shortcomings and ineptitude. I have to say here that Warren (Simons, the editor of Thor) and Dan (Buckley) were absolutely understanding and supportive and gentle with me.  They are gentlemen and scholars and the kind of business-men you want to work with, because through it all they remain human beings, and that's rare.”