"It's going to be the most vicious divorce battle Hollywood's ever seen," Halperin tells ET.
Defying the tried-and-true Hollywood divorce strategy of issuing a joint statement and hashing out differences through mediation, sources tell ET that Jolie, 41, blindsided Pitt, 52, with her divorce filing. Halperin tells ET his sources back that up.
"Everybody, including Brad, was shocked that she lawyered up," Halperin says. "They had a mutual agreement to work it out through mediation, but Angie, being the strong character that she is, she wanted to have the upper hand and come out swinging and make Brad look like the fall guy. Brad has told friends he is crushed, because he doesn't want to trash the mother of his children and he wants to work it out behind the scenes and to put on a brave face."
Jolie set the tone by requesting full physical custody of the kids: Maddox, Pax, Zahara, Shiloh, and twins Vivienne and Knox.
"She's really positioned herself for a career in the U.K.," Halperin explains. "If there is joint custody, Brad could block it and force them to live in the U.S., something she doesn't want. She wants all her children schooled and brought up in the U.K."
For a look back at the pair's fiery relationship over the years, watch the video below.