The sentence sets up a parallel structure joined by "not...but": "not to curb science but to stop war".
For the sentence to be grammatically parallel, both verbs following "not" and "but" need to be in the same form, and since "to curb" is an infinitive, the second verb needs to match it as an infinitive too, "to stop", not a gerund or a construction using "for".
So "to stop war" is the correct completion, keeping the infinitive structure consistent on both sides.