What is the meaning of "in what its commanding officer said was a milestone for himself" in the following sentence?

Question

What is the meaning of "in what its commanding officer said was a milestone for himself" in the following sentence (source),

The missile was fired by the submarine in what its commanding officer said was a milestone for himself.

Does it mean "The missile was fired by the submarine. in the submarine, that its commanding officer said was a milestone for himself" ?

What is the difference between "The missile was fired by the submarine in what its commanding officer said was a milestone for himself" and "The missile was fired by the submarine what its commanding officer said was a milestone for himself" ?

Answer 1

To help us parse the sentence we can remove

[its commanding officer said]

from the sentence because it is only there to add detail that the sentence was an opinion made by the submarine's commanding officer. "Himself" also refers to the same commanding officer, because he said it. Now we have a simpler sentence:

The missile was fired by the submarine in what was a milestone for [the commanding officer].

Next we can find the meaning of "in what":

pronoun: what

2. the thing or things that (used in specifying something).

In this sentence, "what" is the thing that is a milestone.

A milestone is an event, so "what" is also an event. The missile was fired from the submarine during this event.

The missile was fired by the submarine [in an event that] [the submarine's commanding officer said] was a milestone for [the commanding officer].

The full meaning of the sentence is:

• There is an event
• In this event a missile is fired from a submarine
• The submarine's commanding officer said this event was a milestone for himself.