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Policy Analysis

PolicyWatch 3262

Establishing a Response Ratio for Iranian and Proxy Attacks

Michael Knights

Also available in العربية فارسی

February 13, 2020

Enhancing deterrence and protecting Americans in Iraq and Syria requires a more formalized system for rationing out retaliatory strikes at the proper intensity, time, and place.

When U.S. airstrikes targeted Kataib Hezbollah militia personnel and senior Iranian military figures on December 29 and January 3, they were releasing long-pent-up retaliation for a range of provocations by Iraqi militias. Yet while these powerful blows may have injected some caution into enemy calculations, such deterrence is likely to be a wasting asset.

The most proximal trigger for the strikes—the killing of an American civilian contractor during Kataib Hezbollah’s December 27 rocket attack on the K-1 base in Kirkuk—was just one in a series of increasingly risky militia operations against U.S. facilities. Only good fortune has prevented more Americans from dying in attacks conducted since then, including January 8 (when Iranian ballistic missiles struck the U.S. portion of al-Asad Air Base, causing more than a hundred nonlethal traumatic brain injuries), January 26 (mortar strike on the dining hall at the U.S. embassy in Baghdad), January 31 (rockets fired at the U.S. site at Qayyarah West), February 10 (explosive device thrown at a U.S. logistical convoy south of Baghdad), and February 13 (rocket attack on U.S. site at Kirkuk).

The United States has seemingly communicated to Tehran that it will strike Iraqi militias and Iranian targets if any Americans are killed, but this redline has opened up a dangerous gray zone in which Iran and its proxies are emboldened to continue their nonlethal attacks. Besides the fact that such high-risk attacks are destined to result in more American deaths at some point, they will also produce many more injuries if permitted to continue, as seen in the January 8 strike. More broadly, they will limit U.S. freedom of movement in Iraq and Syria, undermining the point of being there in the first place.

This situation is unacceptable—the United States needs a way to deter such behavior even when attacks fall short of killing Americans. When faced with similar challenges in past decades, the U.S. military established reckoning systems that matched the punishment to the crime, with useful levels of predictability, proportionality, and accountability.


In the 1990s, Saddam Hussein’s forces incrementally tested U.S. redlines inside the no-fly zones, using air defense units to target daily American patrols with varying degrees of seriousness. U.S. officials came to realize that they needed a mechanism for continually refreshing and calibrating punitive deterrent responses, not only to keep their forces safe and operational, but also to delegate responses to local commanders while maintaining accountability and proportionality. The solution came in two forms: response ratio and response options.

The response ratio was a reckoning system that ranked different kinds of enemy provocations and then roughly equated them to different U.S. armed responses. The system factored in:

  • Quantity of attacks. The number of attacks within a rolling coverage period (e.g., the past fifteen days).
  • Qualitative filter. For example, random anti-aircraft artillery fire was scored lower than the launch of a deadly SA-6 surface-to-air missile; attacks on vulnerable platforms such as airborne early warfare aircraft scored high as well.
  • Response ratio. The severity of any U.S. kinetic response was set by the numeric reckoning of quantitative and qualitative factors, or by a more informal calculation (e.g., five anti-aircraft artillery attacks might be deemed equivalent to one missile attack).

The response options were sets of pre-vetted targets that could be quickly authorized for retaliatory action. The system included two variables:

  • Choice of targets. Response options were graded. The lowest-level option might be a direct strike on the offending air defense unit. The next level might be an indirect strike on a distant and more valuable set of air defense targets. Higher-level options might include hitting targets outside the no-fly zone or targets other than air defenses. This system gave U.S. commanders the option to occasionally go after valuable strategic targets such as national-level leadership and military sites.
  • Command authorities. Each response option had an authorization level. The lowest option might be approved at the tactical level (i.e., the pilot who was shot at), while more powerful options required approval from the commanding general at CENTCOM or even senior civilian leaders in Washington.

Importantly, the system was classified at secret level, so the enemy did not know exactly how retaliation was weighted or authorized. As such, it could be quietly adapted to fit shifting circumstances in Iraq and political will in Washington. The system also gave U.S. officials the option of withholding a response and adding it to the enemy’s “tab” for a heavier response later, striking at a time and place of their choosing—yet always with a clear internal accounting measure and logic. When applied boldly, as in the northern no-fly zone in 1998, these methods effectively decreased Iraqi attacks on U.S. aircraft.


At least some of the mechanics behind response ratios and options are applicable to the current force protection quandary in Iraq and Syria, where U.S. personnel are once again operating in enemy crosshairs on a daily basis. Indeed, the rapid U.S. response to the fatal December 27 rocket attack suggests that pre-vetted response options were used. Overall, however, the current U.S. posture is predictable and vulnerable, and wholly defensive measures will never be enough to safeguard American troops, diplomats, and contractors. Only by pairing defensive measures with deterrence by punishment can the U.S. government fulfill its duty of ensuring the highest degree of protection.

A new response ratio might factor in:

  • Quantity of attacks. The number of Iran-backed attacks on U.S. forces in Iraq and Syria within a rolling coverage period—say, the past thirty days.
  • Qualitative filter. Rocket and mortar attacks that involve accurate or heavy fire with an evident intent to kill should be scored higher than harassment and aim-to-miss strikes. Strikes on vulnerable targets (e.g., unhardened facilities, aircraft, vehicle convoys) could be ranked higher as well. Use of advanced weapons such as explosively formed penetrator roadside bombs, precision rockets, or ballistic missiles might be scored higher still.

In establishing a response ratio to organize its kinetic responses to such attacks, the U.S. military could consider the following methodology:

  • Response options. The lowest-level options might be counter-fire on rocket teams, as seemingly occurred on January 31 when artillery targeted a rocket team firing on the U.S. site at Qayyara West airfield. Higher-level targets could include militia bases and tactical personnel (such as those struck on December 29) or militia leaders (such as those killed on January 3). Nonlethal near-misses or lethal strikes on personnel close to militia leaders are other potential options, as are cyberattacks. A constant process of rolling target generation, vetting, and approval is already underway, so all that would be required is banding response options into different levels.
  • Command authorities. The authorization level for most or all of these response options should be set quite high, such as the commanding general of CENTCOM or, more likely, senior civilian leaders in Washington. Given the presumed low frequency of necessary strikes, it would not be unrealistic or unduly burdensome to have top-level leaders involved in each case. There would be no rush to strike in most cases, and the system of pre-vetted response options would generally facilitate timely retaliation—certainly rapid enough for U.S. enemies to make the connection between their specific provocations and Washington’s broadly proportionate responses.


Deterrence must be reviewed and refreshed on a regular basis, so any response system would need to be exercised and checked frequently to ensure it is still fit-for-purpose. Washington could then change the gearing of its response ratio at will and secretly. By reducing the pressure to strike back immediately for fear of being seen as turning the other cheek, such a system could mitigate the risk of being lured into retaliatory strikes under dangerous circumstances—especially when enemy forces aim to draw U.S. forces into causing collateral damage. Many components of such a system are already in place (e.g., target vetting, considerations of proportionality), but they are not integrated into a logical self-defense framework.

To break the current cycle, the U.S. government needs a more formal retaliatory system with a balance of mechanical and political features. Otherwise, Iran and its proxy militias will keep pushing the limits of deterrence until they kill or grievously wound more Americans. The January 3 assassination of Kataib Hezbollah leader Abu Mahdi al-Muhandis and Iranian commander Qasem Soleimani was a case of too much retaliation coming all at once, creating a significant risk of unintended escalation and potentially undermining the overall U.S. position in Iraq and Syria. Those who wish for peace should seek a more incremental deterrent system that can keep the temperature down in the U.S.-Iran conflict.

Michael Knights, a senior fellow with The Washington Institute, has worked closely on Iraq security issues since 2003, including a dissertation on U.S. airstrike targeting policies in that theater.