Q:

Is my understanding for the phrase, なくてはならなくなる correct?

The original sentence from the light novel: 

もしもこの作戦が失敗したら、我々は、我々の宿願の為、カルペデイムに戦争を仕掛けなくてはならなくなる。

カルペデイム, Carpe Diem, is a fictional place in the light novel setting.
I've found reference for なくてはならない in both tangorin and this site but so far none for なくてはなら itself. 
For なくては I understand that it means "unless or without" while なら it would mean "if". However, stringing the both together, along with the なくなる phrase, which I understand to be "disappear", in this context, makes me confused.
In context, the whole sentence may be translated as, 
If, assuming that our strategy has failed, for us, the objective of our wish will disappear unless we wage a war in the Carpe Diem. 
Can someone please clarify the usage of the above-mentioned phrases for me?

A:

我々の宿願の為 = 我々の宿願を達成する為
I have never read the novel but I suspect Carpe Diem (= Sieze the day = Enjoy yourself today) refers to some organisation rather than the mere name of a place.
しなくてはならなくなる -> will be obliged to do
Thus I would translate 
もしもこの作戦が失敗したら、我々は、我々の宿願の為、カルペデイムに戦争を仕掛けなくてはならなくなる。 
as 
If this operation fails, we will be obliged to wage war against Carpe Diem in order to achieve our cherished desire.
* added  *
しなくてはならなくなる = しなくては + ならなく + なる <-
しない + て + は + ならない + なる
roughly translates into
not to do + (conjugative suffix) + (particle indicating the preceding word is the subject of the following verb) + not permitted, not acceptable + to become, will
But I recommend you memorise しなくてはならない as "to have to do," since that will greatly simplify the things.

