The original sentence from the light novel:
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?