keisatu ga bouryoku wo tometa.

A natural reading is:

けいさつ が ぼうりょく を とめた。

• 警察 = けいさつ = police
• 暴力 = ぼうりょく = violence
• 止めた = とめた = stopped

So the basic structure is:

police + subject marker + violence + object marker + stopped

が marks the subject of the sentence — the thing that performs the action.

In this sentence, 警察 is the one doing the stopping, so it takes が:

• 警察が = the police / police as the subject

So 警察が暴力を止めた means The police stopped the violence.

を marks the direct object — the thing directly affected by the action.

Here, 暴力 is what was stopped, so it takes を:

• 暴力を = violence as the thing being stopped

That gives:

• 警察が = the police did it
• 暴力を = violence was what they stopped
• 止めた = stopped

止めた is the past tense of 止める, meaning to stop something.

In this sentence, it means:

• stopped
• put an end to
• brought to a stop

So the whole sentence means that the police actively stopped the violence.

This is the classic transitive vs. intransitive difference.

• 止める = to stop something (transitive)
• 止まる = to stop / come to a stop (intransitive)

Here, the police are stopping violence, so we need the transitive verb:

• 警察が暴力を止めた。 = The police stopped the violence.

If you used 止まった, the meaning would be more like:

• 暴力が止まった。 = The violence stopped.

That sentence does not directly say who stopped it.

In this sentence, it is most naturally read とめた.

Why?

• とめる often means to stop something from continuing, to halt, or to bring to a stop
• やめる often means to quit, to give up, or to stop doing something voluntarily

Since the police are stopping violence as an outside force, とめた fits best.

So here:

• 警察が暴力をとめた。 = The police stopped the violence.

警察 usually means the police as an institution or group, not one individual officer.

So this sentence normally means something like:

• The police stopped the violence.

If you wanted to say a police officer, you would more often use:

• 警官 = police officer
• 警察官 = police officer

So 警察 is broader than a single officer.

Japanese usually does not use articles like a, an, or the.

That means 警察 can mean:

• police
• the police

And 暴力 can mean:

• violence
• the violence

The exact meaning depends on context.

In English, you often have to choose the or no article, but Japanese usually leaves that unstated unless it really matters.

Usually yes, but the nuance can vary depending on context.

暴力 generally means:

• violence
• physical force
• violent behavior

In this sentence, it most naturally means violence.

It is a general noun, so it does not tell you exactly what kind of violence unless more context is given.

Japanese word order is more flexible than English, but the verb usually comes at the end.

This sentence is in a very standard order:

• 警察が 暴力を 止めた。

You may also see rearrangements like:

• 暴力を警察が止めた。

This still means roughly the same thing because the particles が and を show each word’s role.

However, changing the order can change the emphasis.

For learners, the safest pattern is:

subject + が / は + object + を + verb

Yes. That would also be natural, but the nuance changes.

• 警察が暴力を止めた。
  Focuses on who did it: It was the police who stopped the violence.

• 警察は暴力を止めた。
  Uses 警察 as the topic: As for the police, they stopped the violence.

Very roughly:

• が = subject focus / identification
• は = topic / contrast / broader statement

Both can be correct, depending on context.

Because 止めた is the plain past form.

• 止める = stop / stops
• 止めた = stopped

So the sentence describes a completed action in the past:

• The police stopped the violence.

If you wanted a non-past form, you would say:

• 警察が暴力を止める。 = The police stop / will stop the violence.

It is in the plain form, not the polite ます form.

• 止めた = plain past
• 止めました = polite past

So:

• 警察が暴力を止めた。 = plain style
• 警察が暴力を止めました。 = polite style

The plain form is very common in:

• dictionaries
• news headlines
• casual speech
• written narration

So it is not necessarily rude; it just is not polite ます style.

By itself, the past tense 止めた usually suggests a specific completed event:

• The police stopped the violence.

If you wanted a more general or habitual meaning, Japanese would usually need a different form or more context.

For example:

• 警察は暴力を止めるべきだ。 = The police should stop violence.
• 警察は暴力を止める。 = The police stop violence / will stop violence.

So this sentence most naturally sounds like one past incident.