This is NOT a GMAT Pill question, but a real LSAT question (However it appears in GMAT Pill

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1) that fear of retaliation is often sufficient to deter would be aggressors.
2) that in order to maintain military deterrence, a nation would have to be believed to have retaliatory power greater than a potential aggressor could defend.
(A) implies that fear of retaliation is required when it uses the words "only if." But the first statement in the stimulus implies that it's "often sufficient."
(B) has the relationship in the second statement backwards. If a nation believes that it's military power surpasses that of others, then it will not be prevented from retaliating - but it may still attack in the first place.
(C) isn't true either. One nation's not attacking doesn't establish that they feared retaliation. Maybe they just got along with the other nation. The stimulus discusses one thing that would prevent a nation from attacking another nation. The stimulus does not discuss the only thing that would prevent one nation from attacking another nation.
(D) must be true. If one nation has a stronger military than any other nation, and military deterrence is achieved by other nations perceiving themselves to be weaker, then the strongest nation should let every other nation know it's strength, so that those other nations will be deterred from attacking.
(E) is not necessarily true. A weaker nation could trick others into thinking that it was stronger and thereby achieve deterrence.