Torto,
Barry Hofstetter answered this question for you in your prior thread: http://discourse.textkit.com/t/help-translating-this-phrase/17509/1
When placeo is used impersonally in the perfect, it often has the meaning of “resolved”, so a more idiomatic translation might be:
The Romans resolved to order Scipio to cross over to Africa to defeat (conquer, vanquish, subdue) Hannibal. ut + the subjunctive and causa + the genitive of the gerund are both ways of expressing purpose.
EDIT: Cross-posted with Seneca