[BluH2o]

I remember the phrase, "necessary war," but I do not recall seeing the word "last" in front of it. Are you sure it said that?

I didn't record the episode, so I can't verify as to 'last' being used in conjunction with 'necessary war'.
However, even if the word 'last' was omitted, it was certainly implied.

[Charles Henrickson]

even if the word 'last' was omitted, it was certainly implied.

Not necessarily. The phrase in the closing credits was picking up on the title of Episode One, "A Necessary War." That title, in turn, was derived from what one of the veterans interviewed said about the reasons for getting into WWII, i.e., that it was "a necessary war."