After Charlie Gibson’s infamous interview with Alaska governor Sarah Palin, much was made of her confused look and pause when Gibson asked her: “Do you agree with the Bush Doctrine?”

But Gibson’s own understanding of the Bush Doctrine was slightly confused:

The Bush Doctrine, as I understand it, is that we have the right of anticipatory self-defense; that we have the right to a preemptive strike against any other country that we think is going to attack us.

A version of this question (and this definition) is bound to reappear in Friday’s foreign policy-focused presidential debate, so we might as well clear this up right now. Gibson was trying to say that the Bush Doctrine equaled the right of the United States to wage preventive war—which means that State A attacks State B because, sometime in the future, State A believes State B could pose an immediate threat. The initial “shock and awe” campaign of 2003’s Operation Iraqi Freedom was a classic example of preventive war.

But by calling the initial attack a “preemptive strike,” Gibson confusingly equated the Bush Doctrine with preemptive war—or the right of State A to attack State B when there is evidence State B is actively preparing to attack State A. In 2006, the RAND Corporation reported (PDF) that most commentators on international law believe preemptive war is only legitimate when an imminent threat exists, “most often a visible mobilization of armies, navies, and air forces preparing to attack.”

The Bush administration’s claims that the Iraq war was preemptive never really matched such scholarly consensus, and were wholly discredited when no weapons of mass destruction were discovered. A precedent for a legitimately waged preemptive war does, however, exist: Israel’s first strike against Egypt in 1967, which began the Six Day War.

Although Gibson’s gaffe may seem like a minor mistake, it’s not, for one big reason: preemptive war is legal, preventive war is not. While it’s unlikely any American official will or would ever be held accountable for waging preventive war in Iraq or anywhere else, RAND does describe the consequences of waging war in contravention of international law:

[T]he perceived illegality of an action affects perceptions of its legitimacy, and that may affect the success of the action through its influence on other nations either as a help or a hindrance. Moreover, even though the perceived illegality of an action alone may not bar that action, striking first when that attack is believed to be illegitimate will entail costs, even if they are difficult to measure …

Understanding the difference between the two rationales for the use of force could help Americans understand how U.S. foreign policy has changed since the preventive war doctrine was enunciated in 2002; it could also help Americans understand the widespread foreign opposition to the invasion and occupation of Iraq. More immediately, though, it could help the public and politicos alike understand future questions about the Bush Doctrine—questions that, as it turns out, can be just as awkward for the interviewer as for the subject.

Matthew Harwood is an associate editor at Security Management and a frequent contributor to The Guardian.