tessyuu ga hazimatta.

It is read:

てっしゅう が はじまった

• 撤収 = てっしゅう
• 始まった = はじまった

So the full sentence is てっしゅうがはじまった.

撤収 is a noun, but it is also a verbal noun (a noun that can combine with する to make a verb).

So you can have:

• 撤収 = withdrawal / pullout / breaking camp
• 撤収する = to withdraw / to pull out / to break camp

In this sentence, 撤収 is being treated as the thing that started.

が marks the thing that is doing or experiencing the action/state of the verb.

Here, the verb is 始まった = started / began.
So 撤収が始まった literally means something like:

• The withdrawal started
• Withdrawal began

The event 撤収 is the thing that began, so it takes が.

This is a very common question.

• 始まる = to begin / to start
  
  • intransitive
  • something begins on its own
• 始める = to start something
  
  • transitive
  • someone starts something

So:

• 撤収が始まった = The withdrawal began
• （誰かが）撤収を始めた = Someone started the withdrawal

In your sentence, the focus is on the event beginning, not on who caused it.

Yes, but it would mean something slightly different.

• 撤収が始まった
  = The withdrawal began
  Focus: the event itself started.

• 撤収を始めた
  = began the withdrawal
  Focus: someone started carrying out the withdrawal.

With を始めた, you usually need a subject from context or explicitly stated:

• 部隊は撤収を始めた。
  The troops began withdrawing.

So your original sentence is more neutral and event-focused.

始まった is the past form of 始まる.

In Japanese, the past form is often used where English might say:

• started
• has started
• sometimes even was underway depending on context

So 撤収が始まった means that, at the reference point in the story or conversation, the start has already happened.

This is very natural in narration:

• 戦闘が終わり、撤収が始まった。
  The fighting ended, and the withdrawal began.

Yes, but the nuance changes.

• 撤収が始まった。
  This simply presents the withdrawal as the thing that began. It sounds natural when introducing the event.

• 撤収は始まった。
  This puts withdrawal in topic position and can sound contrastive or thematic, like:

  • As for the withdrawal, it has begun
  • The withdrawal, at least, has started

So が is usually the more straightforward choice in a sentence like this.

Yes, in the broader sense, someone is presumably withdrawing, but Japanese does not need to state that here.

The sentence talks about the event:

• 撤収が始まった = The withdrawal began

Who is withdrawing is left to context. It might be:

• soldiers
• staff
• a team
• an organization
• residents evacuating from a site

Japanese often omits participants when they are already understood or not important.

It is common, but not in casual everyday situations as often as words like 帰る or 出る.

撤収 has a somewhat formal, operational, or strategic feel. It is often used for:

• military withdrawal
• pulling staff out of an area
• removing equipment
• ending operations at a site
• breaking camp after an event

For example:

• 部隊が撤収した。
  The unit withdrew.
• 機材を撤収する。
  To remove the equipment / pull out the equipment.

So the word can sound serious or organized, depending on context.

No. It is often used in military contexts, but not only there.

It can also mean:

• pulling out of a place
• packing up and leaving a site
• withdrawing personnel or materials
• ending on-site activity and clearing out

For example, after an event or filming session, people might talk about 撤収 when equipment and staff are being packed up and removed.

So the exact English translation depends on context:

• withdrawal
• pullout
• evacuation of personnel
• breaking camp
• packing up and leaving

No, normal Japanese writing does not use spaces between words like this.

It would normally be written as:

撤収が始まった。

Spaces are sometimes added in textbooks or learning materials to make the structure easier to see:

• 撤収 | が | 始まった

So the spaced version is useful for study, but in real Japanese you would usually not write it that way.