Answer:
The correct answer is: Inversion of normal subject/verb/object order
Step-by-step explanation:
Normally, the sentence would be structured as "Let us fulfill our engagements with perfect good faith". However, the author inverts the order to put more emphasis on the condition "So far as we have already formed engagements". This also creates a parallel structure with the following sentence, which begins with "Here let us stop".
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