The Israeli Security Fence
In October 2003, Kerry said Israel’s unilateral construction of a security fence was “a barrier to peace.”
“I know how disheartened Palestinians are by the decision to build the barrier off the Green Line," he told the Arab American Institute National Leadership Conference. “We don't need another barrier to peace. Provocative and counterproductive measures only harm Israelis.”
But less than a year later, in February 2004, he reversed himself, calling the fence "a legitimate act of self-defense," and saying "President Bush is rightly discussing with Israel the exact route of the fence to minimize the hardship it causes innocent Palestinians.”
I wouldn't say that Kerry "reversed himself"; he simply talked out of both sides of his mouth, a technique that the Democrats have mastered by decades of practice (when Republicans try it, it usually backfires).