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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,

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