According to the following sentence:
The bold sentence starts with 以, and it also has 为. So at first I assumed it is 以～为 pattern, which is "regarding A as B" or "taking A as B" in English.
However, I also found that 为首 means "serve as the head", and it should exactly refer to 安倍. And I understand the meaning then.
So is this sentence 以～为 pattern or not? If not, what does the leading 以 mean and work in this context?