Break Off, Break Up

Question

According to this dictionary

definition 2 of "break off" is:

to stop or end suddenly or cause (something) to stop or end suddenly

definition 1b of "break up" is:

to end or cause (something) to end

which suggests that "break off" = "break up".

So, does it mean the a-sentence and b-sentence in the following pairs:

1a "Police broke off the fight."
1b "Police broke up the fight."

2a "The two countries have broken off diplomatic relations."
2b "The two countries have broken up diplomatic relations."

3a "The negotiations resumed soon after they were broken off."
3b "The negotiations resumed soon after they were broken up."

are the same?

Answer 1

There is an important difference not explicitly mentioned in those definitions: the grammatical subject of break off refers to a different agent than the grammatical subject of break up.

The subject of break off was previously participating in or causing the activity that ended. The activity ended because the subject stopped continuing it.

The subject of break up was outside the activity, not part of it.* The subject actively interfered with its continuation, causing it to stop.

For example:

The police broke up the fight.

means that some people, not the police, were fighting, and the police forcibly made them stop fighting. Most likely, the police physically grabbed the combatants, pulled them apart, and restrained them.

The police broke off the fight.

means that the police were themselves involved in the fight, and stopped fighting by their own choice. Perhaps the police were defending a building against a crowd during a public protest, and the police lost heart and decided to walk away and let the crowd do as it pleased. Or perhaps the "fight" was a negotiation to get a raise in pay. Then if the police "broke off" this fight, that would mean that they gave up trying to get the raise, agreeing to continue to work at their present salary.

When people "break off negotiations", this means that they refuse to continue negotiating. They have given up hope of making a deal, or they demand a concession from the other side before they will even continue talking. They have "walked away from the negotiation table."

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*Of course, there is also a different sense of break up, such as sense 2, the break-up of a romantic relationship, in which the participants do end it.

Answer 2

Here, breaking off is usually performed by one or more of the parties involved, while breaking up is generally caused by an outside influence, because both are transitive verbs (they take an object).

I would make the following choices from your examples:

1B is preferred, unless the police were themselves engaged in fighting and decided to end hostilities. Here I assume they came to stop a fight between two other parties.

2A, because the countries were part of the broken off relations.

3A is my first instinct here, because when discussing the unsuccessful end of negotiations, usually one of the parties involved is unhappy and decides to stop talking. Use 3B if some outside group not negotiating arrived and disrupted the proceedings.

Loosely speaking, you can try substituting disrupt for break up and end [their] for break off. Compare the new sentences to get an idea of whether to use up or off.

For the intransitive versions (but still roughly the same definition), the meanings are closer, but still not exactly the same. Relationships or groups which are very close break up, and ones that are at more of a distance are broken off. Looking at alternate definitions in your linked dictionary hints at why this is so: break up means to separate a whole unit into pieces while break off means separate or end.

Things which break up:

• Romantic relationships
• Musical groups or bands
• Governing parliaments or councils

Things which are broken off:

• Negotiations
• Business collaborations
• Relationships with acquaintances

I would consult several other dictionaries to try and get a sense of the differences in usage among the different definitions. Don't expect too much rhyme or reason; English can be funny like that. Here are some samples you can check out: MW (intr. 3, tr. 1), Collins (def. 2), reference.com (up, off; scroll down most of the way or search).