sono koudou ni ha yuuki ga hituyou datta.

The core pattern is:

A には B が 必要だ

This means B is necessary for A.

So in your sentence:

• その行動には = for that action / as for that action
• 勇気が = courage
• 必要だった = was necessary

So the sentence follows the common pattern:

For that action, courage was necessary.

Japanese often builds this kind of sentence around 必要だ, rather than around a verb like need the way English does.

This is a very common learner question.

• に marks the thing that something is necessary for
• は marks that part as the topic

So:

• その行動に = for that action
• その行動には = as for that action / for that action in particular

The は often adds a slight sense of focus or contrast. It can sound like:

• that action, at least, required courage
• when it came to that action, courage was necessary

Without は, the sentence would still be grammatical:

• その行動に勇気が必要だった。

That version is a bit more neutral.
With には, the sentence gives a little more emphasis to that action.

Because with 必要だ, the thing that is needed is usually marked with が.

So Japanese treats 勇気 as the thing that is necessary, not as a direct object of a verb.

Compare the English idea:

• Courage was necessary

That is closer to the Japanese structure than:

• Someone needed courage

So:

• 勇気が必要だ = courage is necessary

This is why が is natural here.

必要 is a na-adjective (also called an adjectival noun).

That means it behaves differently from an i-adjective.

For example:

• 必要だ = is necessary
• 必要だった = was necessary
• 必要です = is necessary (polite)
• 必要なもの = a necessary thing

So in this sentence, 必要 is the main descriptive word, and だった is the past form of だ attached to it.

Because 必要 is not an i-adjective.

An i-adjective makes the past like this:

• 高い → 高かった
• 難しい → 難しかった

But 必要 is a na-adjective, so it uses forms of だ:

• 必要だ = is necessary
• 必要だった = was necessary

So 必要だった is the correct plain past form.

その means that and must come before a noun.

So:

• その行動 = that action

By contrast:

• それ = that one / that thing

You use その when you are directly modifying a noun.
You use それ when the word stands alone.

So these are different:

• その行動 = that action
• それ = that / it

You cannot say それ行動.

It is less natural than その行動には勇気が必要だった.

That is because 必要だ usually connects to the target with に:

• この仕事には経験が必要だ
• 生活にはお金が必要だ
• 成功には努力が必要だ

So the standard pattern is:

X には Y が 必要だ

If you say その行動は勇気が必要だった, listeners can probably understand it, but it sounds less standard and less smooth than using に.

Yes. A more explicit version would be:

その行動をするには、勇気が必要だった。

This means:

To do that action, courage was necessary.

This version spells out the idea of doing the action.

Your original sentence is more compact:

その行動には勇気が必要だった。

That sounds natural and efficient, especially when the context already makes the action clear.

だった makes the sentence plain past.

So it tells us that the necessity existed in the past:

• 必要だ = is necessary
• 必要だった = was necessary

This could mean:

• at that time, courage was needed
• in that situation, courage was required

It does not by itself tell us whether the action actually happened. It only tells us that courage was necessary for it.

It is in the plain style, which is neither automatically rude nor super casual. It is the normal dictionary-style/plain style used in:

• narration
• writing
• conversation with friends or family
• inner thoughts
• explanatory text

A polite version would be:

その行動には勇気が必要でした。

So:

• 必要だった = plain
• 必要でした = polite

Yes. 勇気 means courage in a fairly broad and standard sense.

It often suggests:

• emotional bravery
• mental strength
• the ability to act despite fear

So 勇気が必要だった sounds very natural for something difficult, risky, or emotionally demanding.

It does not necessarily mean dramatic heroism. It can also be used for everyday situations where someone had to be brave enough to act.