This is one of our top strategies for fighting terrorism

Reuters

ISIS fighters on parade

The fight against corruption must become central to our broader strategy against terrorism. From Afghanistan to Kenya, graft fuels terrorist groups like the Taliban and al-Shabaab. The mayor of Mosul called corruption "essential" to the city's fall to Da'esh and now Iraqi forces struggle and die to free the city once more. In case after case, corruption undermines the struggle against terrorism at every level.

First, corruption fuels new terrorist threats. When public officials and businesses can demand bribes with impunity, it shreds the compact between citizens and government and allows terrorists to cast themselves as a better deal. Over 15 years of struggle in Afghanistan, we've seen the Taliban replenish fighters by railing against government corruption. The Department of State's research on Africa found that personal experiences with corruption were significantly linked to political violence, including violent extremism.

Second, corruption helps existing terrorist groups sustain operations and infiltrate new areas. In Mosul, smuggled oil enriched Da'esh before they seized the city. In Eastern Europe, foreign terrorist fighters have bribed border guards on their path to jihad. Across West Africa, we've heard reports of soldiers selling weapons on the black market to Boko Haram, or even tipping off al-Qa'ida about future troop deployments for a fee. As one currently imprisoned al-Qa'ida fighter said, "Thank god, Mali is a very corrupt country."

Finally, corruption saps the government's ability to combat terrorist threats and ensure security. When President Buhari of Nigeria came to office, he inherited an army hollowed out by decades of graft that was wholly unprepared to confront Boko Haram. When Prime Minister al-Abadi assumed power in Iraq, he found 50,000 "ghost soldiers" on the payroll draining precious resources from the fight against Da'esh. By reducing the combat effectiveness of critical foreign partners, corruption reduces the impact of our foreign assistance and increases foreign dependence on U.S. military power – exposing our soldiers to even greater risk.

For all of these reasons, Secretary of State John Kerry has called the fight against corruption a first-order national security priority.

Today in Kenya, the U.S. is making a concerted effort to fight both terrorism and corruption. There, as in many other places, corruption feeds and enables terrorism. Smuggled rice and sugar enriches al-Shabaab, and its fighters repeatedly invoke corruption and alleged abuses by Kenyan security forces to recruit new fighters. So as we intensify counter-terrorism operations against the terrorist group, we are also ramping up the battle against corruption.

Last July, the U.S. and Kenya made an unprecedented joint commitment to partner on over 40 major actions to combat graft. We've since helped to create a special anti-corruption investigative unit, which made its first arrests this October. We've helped Kenya develop mandatory ethics training for Kenyan public servants and have partnered with local police to promote accountability in their ranks. On top of that, we are collaborating with Kenya to develop tools to fight money laundering, improve transparency in procurement systems, and strengthen legal protections for whistleblowers. And we encourage Kenya's participation in the Open Government Partnership, a platform for governments and citizens to share lessons in promoting transparency. Though Kenya's fight against corruption has only begun, these steps speak to the breadth of actions we can take with partners across the globe.

For years, the world has looked at corruption as a threat to economic growth and human rights. But we have underestimated the threat – corruption is far more dangerous than many realize, and it is time to confront it with the urgency, intensity, and resources it deserves. In doing so, we can help build societies that are not only more transparent and accountable, but also more secure and safe from the threat of terrorism.

Commentary by Sarah Sewall, the Under Secretary of State for civilian security, democracy and human rights. Follow her on Twitter @civsecatstate.

For more insight from CNBC contributors, follow @CNBCopinion on Twitter.