Consider the following hypothetical: It is 10:00pm in Times Square, New York City. The “city that never sleeps” is bustling with tourists, Broadway shows, street performers, and exhausted professionals making their way home to catch a few hours of sleep before returning to the office. At once, a loud explosion is heard in the distance, followed by a stream of “bangs”: rapid gunfire. Several individuals emerge from the shadows in various locations around the area carrying semi-automatic weapons, loaded to the teeth with ammunition. They begin firing. One of the armed individuals charges into the Minskoff Theater at the edge of Times Square, where a packed audience is enjoying the final scene of “The Lion King.” Gunfire echoes throughout the hall. Video surveillance taken from the scene confirms through facial recognition software that at least two of the armed individuals are known ISIS operatives. Other areas of New York City are experiencing similar attacks. An explosion shakes Wall Street as an individual detonates an explosive vest, instantly killing himself and seven others. Seconds later, another explodes in Central Park. By the end of the night, at least 130 individuals lie dead in the streets of NYC. Several of the armed individuals, all confirmed ISIS operatives at this point, manage to flee. They return to several safe houses across New York, where they join at least 50 other ISIS operatives who remain in hiding, waiting for their turn to strike. As the United States responds to this scenario, which legal regime applies?

Roughly two months have passed since 19 ISIS-motivated terrorists attacked the streets of Paris, leaving 130 dead. As the world mourns this tragedy, one cannot help but think: It could have been us. Though concerning, this notion sparks an interesting question: If ISIS operatives infiltrated the United States in a Paris-like attack, as described above, which legal regime would apply: human rights law (HRL) or the law of armed conflict (LOAC)? In other words, if “Paris” happened in the United States, would the rules of engagement center on law enforcement principles or the laws of war? At first blush, answering this question may appear simple in light of previous terror-related incidents within the United States. HRL remains the likely standard. However, there are significant differences between past isolated terror attacks and those like Paris that make answering this question more difficult than it may seem. When it comes to ISIS in the US, an argument may be made that either legal regime — or both — actually apply.

Argument #1: Human Rights Law Applies

Before making the argument that HRL applies in a Paris-like scenario, we must first understand a bit about human rights law. Though aspects of HRL date back to antiquity, contemporary legal standards emerged in the wake of World War II with the creation of the United Nations and subsequent Universal Declaration of Human Rights. Since that time, international human rights law has materialized as the bedrock rule of law for all civil societies. It rightfully provides the foundation for law enforcement activities within the United States and peacekeeping operations worldwide. Similar to our own Declaration of Independence, the Universal Declaration of Human Rights expresses that all individuals enjoy, among other things, an inherent “right to life, liberty, and security of person.” Implicit within this concept is the right to due process of law. In a law enforcement context, due process comes, for example, in the form of a criminal trial, where suspected individuals are presumed innocent until proven guilty by reasonable and competent evidence beyond a reasonable doubt. In other words, HRL equals criminal detention, crime scene investigation, evidence collection, and criminal proceedings. Additionally, in an HRL scenario, lethal force exists only as an option of absolute last resort.

A strong argument may be made that HRL remains the appropriate standard if a Paris-style attack happened in the US. For some, the hypothetical scenario is not a depiction of war; it is a criminal atrocity. Terrorists, such as those in the recent San Bernardino attack, are not combatants; they are criminals. As a result, it remains a law enforcement situation. Such has been the United States’ response to domestic acts of terror throughout recent history. Timothy McVeigh was tried, convicted, and sentenced to death as a result of the Oklahoma City bombing. Dzhokhar Tsarnaev was sentenced to death this past May for his role in the Boston Marathon bombing. Nadal Hassan received the death penalty via military court-martial for killing 13 and wounding 32 others in the Fort Hood massacre. These past examples are an appropriate US response to domestic terrorism. Moreover, from a policy perspective, our nation is founded on principles embedded within human rights law. The United States helped birth the notion of HRL when Thomas Jefferson, inspired by philosophers like John Locke and France’s own Jean-Jacques Rousseau, drafted the Declaration of Independence. At our core, it is who we are and it is where we should stay.

In addition, there are many practical challenges with applying the LOAC to a domestic scenario. Undoubtedly, responses to terror within the homeland fall on the expertise of the FBI, Department of Homeland Security, local law enforcement, and numerous other agencies responsible for protecting our nation. Are we to assume that such individuals should forego their foundational training in HRL and adopt the rules of war? Are members of law enforcement even considered lawful combatants under the laws of armed conflict? In the event law enforcement officials find themselves inadequately trained or equipped for this form of engagement, does that mean our armed forces take the lead? Would military efforts within the domestic arena violate any aspects of the Posse Comitatus Act, which limits the military’s ability to support civilian law enforcement activities? And the questions go on.

Further challenges may emerge if any ISIS operatives are known United States citizens. Many within the legal community were outraged by the 2011 targeting of US citizen (and al-Qaeda operative) Anwar al-Aulaqi in Yemen. How much more concerning would it be if such strikes occurred, not half the world away, but in our own backyard? Additionally, under current case law, an individual’s lawful presence in the United States provides them with certain constitutional protections, arguably regardless of their intentions while on US soil. The legal and practical implications of utilizing the LOAC on American soil boggle the mind, leading many to believe that we should err on the side of legal caution and rely on the rule rather than the exception: human rights law.

Argument #2: The Law of Armed Conflict Applies

An argument that the LOAC applies also requires a basic understanding of the law. Certain principles of war existed long before the births of renowned military theorists such as Sun Tzu and Carl von Clausewitz. Modern LOAC principles, however, solidified as international law around the same time as the creation of the Universal Declaration of Human Rights in what became the Geneva Conventions of 1949. (Though other aspects of the LOAC preceded the Geneva Conventions, we will keep the history brief for purposes of this post.) Suffice it to say that, unlike HRL, the LOAC does not emphasize nor require the kind of process we traditionally require in the HRL context. War is not focused on preparing a case for the courtroom. Evidence comes in the form of intelligence collection. And when it comes to engaging lawful targets (in this case, enemy combatants directly participating in hostilities or possibly serving in a continuous combat function), unlike HRL, lethal force is not a method of last resort. In war, if the LOAC so authorizes, you shoot first and shoot to kill.

An interesting argument may be made that the LOAC applies to a Paris-like hypothetical. The situation posed by ISIS and the United States likely represents a non-international armed conflict, or NIAC (i.e., a conflict between a State and a non-State actor), thus meriting application of the LOAC. The International Criminal Tribunal for the former Yugoslavia, in its Tadić decision, asserted that a NIAC exists “whenever there is … protracted armed violence between governmental authorities and organised armed groups or between such groups within a State.” This legal guide serves today as the international benchmark for this issue. One could argue that the interaction between the United States and ISIS is that of “protracted armed violence.” Those opposing this position may assert that, while the United States has engaged in protracted violence against ISIS via airstrikes since 2014, ISIS has not reciprocated with actual violence against the US in a “protracted” manner. Some may find that a few isolated murders and a single domestic incident in NYC does not get us there. This position is likely overcome, however, when considering the totality of circumstances involving ISIS. The consistent violence asserted against the international community, continuous threats against the United States, direct (albeit isolated) attacks against various United States citizens over the past two years, and inclusion of the hypothetical scenario likely satisfies the international law standard for “protracted armed violence,” thereby triggering a NIAC, and thus, the law of armed conflict.

Additionally, it is important to emphasize that the United States already acknowledges that we are in a state of war with ISIS — even absent the hypothetical “attack” on New York City. Such statements have come from the Secretary of Defense, Secretary of State, and White House staff. In his address to the nation last month, President Obama again confirmed that we are at war with ISIS. Moreover, we have engaged in LOAC operations against ISIS in Iraq and Syria since the fall of 2014. Therefore, the position seems clear. According to our own nation’s leaders and current operations in the Middle East, we are at war with ISIS.

That ISIS brought the battlefield to our backyard in the hypothetical does not change the fact that the LOAC may apply domestically as it already does in Iraq and Syria. When an armed conflict exists, even in a NIAC scenario, the LOAC applies in the full territories of both parties. As noted by the Tadić court, the LOAC “continues to apply in the whole territory of the warring States or, in the case of [a NIAC], the whole territory under the control of a party, whether or not actual combat takes place there. The reach of [the LOAC] therefore extends far beyond the immediate ‘area of operations’ or zone of battle.” One cannot limit the applicability of the LOAC to territory controlled by only one party, or in this case, argue that the LOAC applies in Iraq and Syria but not the United States. In an armed conflict — even a NIAC — the LOAC applies in the full territories of both sides of the war. That the United States has not engaged in prolonged armed conflict on its mainland since the Civil War does not change this sobering reality. Therefore, any argument suggesting that the LOAC cannot apply in the domestic environment simply misunderstands the nature — and law — of war.

Two other factors distinguish the proposed scenario from past terror incidents within the United States, further advancing the LOAC argument. First, consider the aggressor. While one may rightfully acknowledge that previous individuals who engaged in domestic terrorism received due process under HRL, the hypothetical scenario differs in a significant way. Since 9/11, domestic terrorists have remained, at best, lone wolf actors engaged in individual criminal plots. San Bernardino provides the most recent, tragic example. In contrast, the hypothetical aggressors are ISIS operatives, directly connected to the very group on which the United States has declared war. Second, consider the operation. Unlike previous terror events planned by lone wolf attackers, ISIS planned and executed the proposed attack scenario — as they did in Paris. These two details distinguish the existing threat from previous terror incidents, further supporting an argument that the LOAC, not HRL, applies — even on US soil.

The Complication: Both Legal Regimes Apply

While this post presents each side of the argument as a “zero sum game,” it is important to note that both legal paradigms may apply in a given response scenario. This is because the executive’s decision to engage ISIS under the LOAC paradigm does not remove the underlying HRL component. As is always the case, HRL continues to apply to some degree. This is not a far-fetched idea in light of the current debate regarding the relationship between HRL and the LOAC. Professionals within the academic and operational law communities have devoted decades to defining the appropriate HRL/LOAC relationship. The complexity of the contemporary battlefield within the “war on terror” further amplifies this issue. In many ways, the ICRC’s position remains the international standard, regarding the LOAC as a form of lex specialis (i.e., the exception to the general rule of HRL). Under this approach, the LOAC operates in addition to the HRL bedrock, trumping any matters where the two regimes directly conflict while ensuring that HRL continues to apply to matters left unresolved by the LOAC. This issue, however, is far from resolved. To read further regarding the ongoing HRL/LOAC debate, see (just for example) here, here, here, and here. The divergent approaches within this discussion, however, illustrate the international community’s inability to reach a full consensus on the relationship between HRL and the LOAC — something that likely deserves considerably more attention in light of current events.

What is fascinating is how a Paris-style attack on the US homeland would likely shape the debate. Identifying the appropriate relationship between HRL and the LOAC within the domestic arena may prove difficult, especially in light of the fact that our wars traditionally occur abroad. The ICRC’s current position provides little, if any, assistance. Some scenarios may require use of force under the LOAC paradigm while others may appropriately remain within the HRL regime. Choosing the appropriate response to any given scenario will likely require an analysis of the threat level, necessity for military engagement, and risk to civilian life — to name just a few considerations. However, countless questions remain. For example, if HRL provides the foundation, which type of scenario would shift the rules of engagement from HRL to the LOAC — or vice versa? Would a transfer to the LOAC require a complete breakdown of law enforcement capability? Does the LOAC require something akin to an ISIS invasion? If so, how many operatives would it take? And these questions merely break the surface. While finding answers to these questions goes far beyond the boundaries of this post, it highlights the true complexity of the situation we may be dealing with in the future.

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As John Stuart Mill once wrote, “War is an ugly thing.” It is even uglier when it occurs on US soil. As ISIS continues to gain momentum and an international following, attacks on the homeland become a real possibility, if not, as one former acting Director of the CIA recently suggested, a foregone conclusion. For some, extending the full protection of due process rights to invading ISIS operatives would be detestable and, according to the LOAC, unnecessary. For others, due process is one of our nation’s most cherished values and continues to apply in such scenarios. Regardless of one’s position, we must confront the reality of the situation now and begin to answer this question in order to be prepared for if — or when — “Paris” occurs at home.

The views expressed in this article are those of the author and do not reflect on the official policy or position of the United States Air Force, Department of Defense, or US Government.