Though newspaper headlines, such as “Terror Returns” in USA Today the day used the word, President Obama at first did not call the Boston Marathon bombings an act of terrorism. (Arno Burgi/ARNO BURGI/PICTURE-ALLIANCE/DPA/AP IMAGES)

Juan C. Zarate served as deputy national security adviser for combating terrorism from 2005 to 2009 under President George W. Bush. He is a senior adviser at the Center for Strategic and International Studies and the author of the forthcoming “Treasury’s War: The Unleashing of a New Era of Financial Warfare.”

President Obama came under some criticism this past week when he initially refrained from using the word “terrorism” to describe the Boston Marathon bombing. He referred to it as a “senseless loss” and vowed that “we will find out who did this, and we will hold them accountable.”

But no “terrorism,” even as graphic images of maimed victims filled the airwaves and the Internet.

Why not? To many, “terrorism” would seem the most common-sense way to describe the heinous bombing. Why would the president wait until Tuesday morning to refer to the tragedy as an “act of terrorism”?

There are several reasons that any president — and particularly Obama — would rightfully exercise restraint in using the word initially.

The term has legal and national security implications. The language of terrorism is defined and consequential. It also has political ramifications for an administration that has sought to play down the idea of a broad “war on terror” and has used the idea of a receding threat to explain the winding down of U.S. military involvement overseas.

This debate over “terrorism” is familiar. After the attacks in Benghazi, Libya, last Sept. 11, there was much public disagreement about whether to label the attacks on the U.S. diplomatic outpost and annex as “terrorism.” Certainly, any leader wants to avoid getting ahead of the facts as an investigation unfolds. Time and again when I was serving in the White House under President George W. Bush, we saw the “fog of war” color what we knew immediately after major terrorist incidents such as the 2005 London transit bombings and the 2008 attacks in Mumbai. Of course, with the identification of the two Boston suspectsand the dramatic manhunt that followed, we’ve learned much more about the alleged perpetrators. But in such early moments, the president, quite appropriately, wants to evoke a sense of calm and encourage national resilience.

There are specific legal definitions of “terrorism,” found in several variations in U.S. law. In general, terrorism is not just an act of violence but one conducted with a particular purpose in mind: to intimidate a population or to coerce or affect a government’s policy. Such acts are often committed by designated foreign terrorist organizations such as al-Qaeda. Without more clarity, a president may not want to draw conclusions about the intent or purpose of an act of violence.

The use of the term “terrorism” also has direct national security implications. With this label, there is a recognition that our national sovereignty and citizenry have been attacked, whether from within or by external actors (or both). This suggests that there could be related attacks to follow and that a response to prevent future strikes or to punish the perpetrators will be necessary. Because we often think about terrorism in terms of war, this has potential military and lethal consequences.

There are also clear political consequences to labeling something an act of terror. Questions emerge immediately about why our counterterrorism and intelligence systems, built up over a decade, were unable to prevent the attack. Inevitably, the potential political ramifications color the language used.

For the Obama administration, the labeling matters fundamentally. This administration has often pointed to the killing of Osama bin Laden and the decimation of “core” al-Qaeda in the border region of Afghanistan and Pakistan as evidence that its counterterrorism policies are effective. This mind-set has helped frame key political issues, such as the nature and timing of the U.S. withdrawal from Afghanistan.

A terrorist attack on U.S. shores, whoever is responsible, damages that narrative. The Boston bombing demonstrates that terror of some stripe can still threaten the homeland. It is difficult to argue that the tides of war are receding if there are terrorist attacks in the United States and al-Qaeda affiliates thriving from West Africa to Central Asia.

When the Obama administration came into office, it constrained the language of the counterterrorism campaign from a “war on terror” to a war on al-Qaeda. This new lexicon was meant to help focus our efforts on the most direct threat to the United States and to avoid a broad campaign against multiple actors or a global phenomenon. But it ignored the reality that the terrorist threat was already metastasizing. Viewing the threat solely through the al-Qaeda lens — at a time when the danger has grown beyond that — can distort our understanding of terrorism.

The terrorist threat of 2013 is not the same one we faced on Sept. 11, 2001. Al-Qaeda has morphed from a hierarchical organization into a terrorist hydra. The group’s new franchises — in places such as Yemen, Somalia and North Africa— have served as drivers of local insurgencies and as platforms for terrorist innovation, connections and threats to the United States.

The core of al-Qaeda has been decimated, but its leadership, including Ayman al-Zawahiri, is the ideological center of a violent Sunni extremist movement that aspires to energize and unify the group’s various branches and resurrect its relevance after the Arab Spring. Al-Qaeda’s strategy now revolves around bleeding the United States with a thousand cuts — inspiring followers to “attack in place.” This effort comes alive in regional Sunni terrorist organizations and individuals radicalized online. “Lone wolves” infected with this ideology have been caught in FBI sting operations over the years and remain a major concern for counterterrorism officials.

But terror threats do not emanate just from groups linked to al-Qaeda. Other groups, such as Lashkar-e-Taiba in Pakistan, represent serious threats, while Hezbollah and its state sponsor, Iran, remain willing to attack across borders, as seen in the bombing of a bus in Bulgaria last year. We also see the melding of illicit networks — such as drug-trafficking organizations and terrorist groups in West Africa — for profit and mutual support, as well as persistent domestic threats from anti-government and anarchist movements that can rear their heads with great ferocity.

Undeniably, the attack in Boston was a terrorist act, which is why the president ultimately opted for a common-sense definition and called it what it was. But the debate over labeling reflects a more fundamental question of how we deal with the changed and shifting terrorist danger in 2013.

The language of terror has consequences.

There is a debate unfolding about whether Congress should issue a new authorization for the use of military force to update the one it provided Bush on Sept. 18, 2001,reflecting that today’s threats are not necessarily tied to the Sept. 11 attacks. Who can or should we target with lethal force? Which terrorism suspects may be held indefinitely? Do we see a terrorist insurgency in Yemen or Nigeria, or a drug cartel using terrorist tactics, as a threat to the United States, or are our enemies simply those planning imminent attacks against U.S. interests at home or abroad? Does the ideology of al-Qaeda, as it evolves in places such as Syria and North Africa, define the outer limits of threats against America?

The way we talk about terrorism shapes the response to these questions. We have yet to grapple with the hardest of them.

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