Final answer:
A preemptive strike is a military action taken to prevent an anticipated threat by acting before the adversary. Examples include the 2003 Iraq War and the historical shift in U.S. foreign policy towards more preemptive military actions.
Step-by-step explanation:
What is a Preemptive Strike?
A preemptive strike is a military action initiated to neutralize an imminent threat by taking the offensive. It is a proactive measure to prevent an anticipated problem from adversaries that are considered to pose a direct and immediate danger. The rationale behind a preemptive strike is to gain the advantage by acting before the enemy can carry out their own attack against one's nation or interests. In the context of international law and practice, the legitimacy of a preemptive strike is contingent upon the existence of an imminent threat.
The decision to engage in a preemptive strike has historically been debated within the sphere of national security strategy and foreign policy. While traditionally the United States has responded retaliatory upon direct aggression, as seen after the Pearl Harbor attack, there have been instances where preemptive actions have been taken to curb perceived threats. Notable example of this would be the 2003 invasion of Iraq, initiated under the notion that Iraq possessed weapons of mass destruction that could be used against U.S. interests.
Strictly speaking, and referencing the choices provided, the correct answer to the question would be both B) A proactive measure to prevent an anticipated problem, and to a lesser extent, D) A defensive action only, since preemptive strikes are typically justified as a form of self-defense.