To break a promise; to go back on one's word or fail to honor an agreement.
彼は何度も約束を破ったので、誰も信用しなくなった。
He broke his promises many times, so no one trusts him anymore.
約束を破るのはよくないと子どもに教えた。
I taught my child that breaking promises is wrong.
もし彼が約束を破るなら、関係を見直さなければならない。
If he breaks his promise, we'll have to reconsider the relationship.