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The roller coaster of U.S.-North Korea relations revs up again this week when President Donald Trump meets for a second time with Kim Jong Un in Hanoi, Vietnam.

Trump has gone from threatening North Korea with "fire and fury" to saying he and Kim are in love. Kim once called Trump a dotard and now is pledging to denuclearize his nation and turn his energy toward developing his country's economy.

The ups and downs in the relationship have rattled Americans and allies alike. And the leaders' erratic history is contributing to uncertainty heading into this week's talks in Vietnam.