IF STRATEGY is the art of rethinking the possible, then the time for strategic innovation against what the U.S. military terms violent extremist organizations (VEOs) is now. The American-led air war in Iraq and Syria may have shown some progress against the Islamic State and other VEOs, but the VEOs and their sympathizers have hit back with attacks in France and brutal beheadings of journalists and aid workers. Frustration is growing in Congress as the traditional tools of American power fail to produce decisive results. But what can those on Capitol Hill do?

More than they realize. An overlooked clause of the Constitution empowers them to take the fight to VEOs in a way the president cannot. Invoking this clause would help Congress restore its faded prerogatives in national security while hitting America’s extremist enemies harder—and it would be cheap. Under Article I, Section 8, Congress “shall have Power . . . to declare War, grant Letters of Marque and Reprisal, and make Rules concerning Captures on Land and Water.” American legislators should now consider resurrecting this long-forgotten but constitutionally enshrined strategic alternative by issuing letters of marque and reprisal.

Historically, these letters provided the legal authority by which naval warfare could be conducted by private individuals, commonly known as privateers. Even before the Constitution was ratified, many colonial governors (and the infant Continental Congress) issued letters to promote raiding against Britain’s economic assets in North America during the Revolutionary War. This approach was deemed necessary because the young United States lacked a navy of sufficient size to confront its early enemies. The letters expeditiously authorized a much-needed military capability to wage war when other tools weren’t readily available. The situation is similar today, when political choices and fiscal barriers hobble the deployment of uniformed American and coalition ground forces into Iraq and Syria. There are insufficient ground forces to alter the territorial status quo the Islamic State has created. By authorizing private forces with letters of marque, Congress will begin to address the pressing need to engage in ground combat against VEO militants.

ARE LETTERS of marque legal? In 1856, fifty-five governments ratified the Paris Declaration Respecting Maritime Law, which banned states from commissioning privateers. The United States was not a signatory of the declaration. However, as its official naval assets increased, the use of letters faded from America’s strategic consciousness. Despite this historic shift, the United States has signed and ratified no international treaties that supersede Congress’s authority to both modernize and reissue letters.

Congress can again participate in shaping national strategy by issuing letters to any number of the private military corporations (PMCs) that have expressed a willingness to go to war by putting private “boots on the ground.” PMCs were integral force multipliers in Iraq, Afghanistan and elsewhere, freeing regular troops for higher-priority tasks. Unfortunately, like the privateers of yore, they have a mixed reputation, in part due to incidents in which they have operated beyond the scope of their contracts. There is also a public perception that they exist solely for profit. However, as VEOs proliferate and spread carnage to more and more countries, letters issued by Congress to private companies (American or international) might offer strategic advantages in flexibility, speed and cost control. Ultimately, they could produce results on the battlefield, with private troops doing the fighting. Bring back the privateers.

WHO WOULD these privateers be? Taking a page from the all-volunteer Rough Riders of the Spanish-American War and the famed Abraham Lincoln Brigade of the Spanish Civil War, PMCs could recruit socially concerned individuals to go fight for a good cause abroad. In doing so, American and European Muslims who are offended by VEOs’ blood-soaked blasphemies would have a productive venue to help defend their faith in Iraq and Syria. In addition to Muslims motivated to fight to preserve the reputation of their faith and restore peace to their coreligionists’ homelands, many veterans of America’s recent wars may be drawn to employment with PMCs out of a sense of duty and a longing to finish the fight begun on September 11, 2001. By late 2014, news outlets reported that the first American and Canadian veterans had arrived as private citizens to fight alongside the Kurds in northern Iraq. Unconventional? Yes. But PMCs and modern privateering are ideas worth embracing if Congress is seeking new options to augment the growing international coalition against the scourge of VEOs like the Islamic State and Al Qaeda.

How would this work? Each letter would spell out in plain terms the fiscal incentives and limits for the contractor. For instance, any oil seized from VEOs would be theirs to sell. Any territory, oil refineries or other valuable infrastructure PMCs liberate could be transferred back to local authorities after their value was assessed and a payment made for services rendered. There are already clear signs of interested privateers gathering resources to operate in Syria and Iraq. Erik Prince, the founder of the well-known PMC Blackwater, commented last fall that “if the old Blackwater team were still together, I have high confidence that a multi-brigade-size unit of veteran American contractors or a multi-national force could be rapidly assembled and deployed to be that necessary ground combat team” in Iraq and Syria. PMCs such as Prince’s could be issued letters by Congress to fill specific requirements the U.S. military cannot or will not meet immediately. Fueled through the incentive of profit, the PMCs would then make their way to the area of operations through their own logistics and begin to conduct operations as stipulated under the letter.

According to recent news reports, the Islamic State will soon tender its own coinage using precious and semiprecious metals (gold, silver and copper). This news should further entice private-sector entrepreneurs to conduct operations against the war-making capabilities of this particular VEO. Seized Islamic State currency could be used in two ways to empower a letter-bearing PMC. First, if the currency is minted in accordance with international standards, it will have very real value in the global commodities market. Thus, confiscated currency is a reward in itself, and it can be used directly to perpetuate ground operations wherever the coins are used as legal tender. Alternatively, a clever PMC might also have a great deal to gain from successfully seizing minting molds (or fabricating their own) to produce counterfeit coinage. A letter could be issued to allow such a practice. The result would surely bring the terms “wooden” and “plugged” nickel (or dinar) back into vogue, and see the Islamic State’s dream of a non-Western currency systematically eroded. Indeed, these tactics of political-economic warfare might have a greater impact than any smart bomb—they would help delegitimize the Islamic State from within.

In Iraq and Syria, PMCs would be beneficial in providing the “on the ground” auxiliary component to the coalition-led air strikes to reduce the Islamic State’s war-making capabilities. They would do so specifically by reducing its revenue sources (such as oil) along key lines of communication where the Islamic State currently enjoys its freedom of movement. Though other governments may not yet issue letters as a regular policy, they could do so. For instance, the Islamic State’s gains have led the Iraqi government to seek foreign fighters of its own. One U.S. firm is rumored to have been offered up to $1,750 per man per day to conduct combat operations in Iraq. This is a far cry from the days of Operation Iraqi Freedom, where PMCs were restricted to “security operations” but no combat. Other countries could issue their own letters: most notably France and the United Kingdom, which are both close U.S. allies and UN Security Council permanent members.

GROUND COMBAT is only one domain for which letters could be issued to support the conduct of operations against VEOs. Cyberspace is a vast virtual sanctuary from which terrorist organizations plan, mobilize, recruit, advertise and conduct logistics using websites, chat rooms and social-media networks. However, even though cyberspace has served to expand the number and capacity of modern VEOs, it is not an impenetrable or absolutely secure domain. For instance, after the Charlie Hebdo attacks, outraged “hacktivists” from the group Anonymous (normally considered anarchists by state authorities) reportedly launched private cyberattacks on the Islamic State. We are not calling for Anonymous to be issued a letter, but there are hundreds of legitimate technology companies (e.g., Google, Cisco and IBM) that have both the skill and the patriotism to take on VEOs in cyberspace. However, these technology titans could only do so prudently if the economic incentives and legal authorities were clearly articulated in a letter. In order to ensure further legal oversight, a two-hundred-year-old mechanism could be revisited: prize courts. Prize courts ensured that privateers were not rewarded beyond the scope of the authority outlined in their letter. The same juridical concept could be applied today.

Have individuals incentivized by profit ever filled their pockets with other people’s Bitcoins or bank balances? Or, worse yet, have they filled their databases with credit-card or bank-account numbers? Of course—hackers do it all the time. Each time a security system is breached at a major retailer or bank, account information is the focus of the attack. The entire fraud-protection industry is built around safeguarding personal and account information with the aim of defeating these profit-seeking criminals.