Britney Spears' Ex Jason Alexander Slams Kevin Federline for New Tell-All

Kevin Federline's new book, "You Thought You Knew," spills salacious details about his ex Britney Spears' shortcomings during their relationship ... but Britney's first husband is coming to her defense.

Jason Alexander -- who was married to the "Toxic" singer for 55 hours before their annulment -- took to Instagram to set Kevin straight.

Jason wrote, "You don't get to tell half the story. Not when you were in the middle of it."

He continued, "It’s easy to point fingers now, to talk about Britney’s struggles like you were just some innocent bystander watching it all happen. But that’s not what went down."

In fact, Jason alleges KF was actually feeding off of "the drugs, the alcohol, the chaos" that was Britney's life at the time.

In an excerpt from the book, obtained by USA Today, Kevin accused Britney of cheating on him with a female dancer early on in their relationship during a tour stop in Amsterdam. As rough as that sounds, he's not getting any sympathy out of Jason.

Jason said, bluntly, "Let’s be honest -- you became a millionaire off of her pain."

Jason clarified he doesn't have a problem with Kevin giving his side of the events that took place in their relationship ... he just wants him to hold himself accountable for his role in things as well.

"Don’t pretend you were the victim or the hero," he wrote. "Because those of us who were really there -- who actually cared about her, not the fame -- we remember how it really went down."