Official Explanation
Split #1: We have an independent clause, then the word "but": after this, we need another independent clause with Genghis as the subject. He must be followed by a full verb, "posed," not the participle "posing." Choices (A) & (B) make this mistake.
Split #2: Right after the word "but" we have the opening of a "because" clause with the subject "Attila's home base." This subject also needs a full verb. We need "was much closer." Choice (C) omits the verb.
Split #3: Finally, we have an ambiguous comparison in (E). For simplicity, let's abbreviate: X = "Attila's home base, in modern Hungary," Y = "Western Europe," and Z = "Genghis' Mongolian headquarters." Using these abbreviations, (E) says:
X was much closer to Y than Z
This could mean:
X was much closer to Y than it was to Z
or
X was much closer to Y than Z was close to Y.
We can tell from the prompt that the second meaning is the intended one, but (E) has been so shortened that it no longer allows us to resolve this ambiguity, and ambiguity is always wrong! Choice (E) is incorrect!
The only possible answer is (D).