Sean Penn‘s crusade to save the world cost him his marriage.

That’s the REAL story why actress Robin Wright Penn pulled the plug on her marriage to the Oscar winner after 11 years.

The hotheaded actor-turned-activist lost himself in his career and his political causes intensified after the tragic death of his brother Chris, who died from heart problems and drug complications in 2006, sources told The ENQUIRER.

“Sean was never the same after Chris died — and he was also never home,” said a longtime family friend.

“He was either off meeting President Hugo Chavez in Venezuela, traveling to Iran as a journalist, helping Katrina victims in New Orleans or making a movie. Robin put up with a lot during their years together, but in the end she found herself stuck as a single parent in an empty marriage.”

She finally decided to end the marriage.

Click here to see the court papers. Plus: Pick up the new issue of ENQUIRER for more details!