Iraqi security forces patrol Fallujah, Iraq, on June 28 after the city was liberated from the Islamic State. (Karim Kadim/Associated Press)

Michèle Flournoy is chief executive and founder of the Center for a New American Security. Ilan Goldenberg directs the center’s Middle East security program.

There are two theaters in the conflict with the Islamic State in Iraq and Syria, and they are not defined by international borders. The first is “ISIS-stan” in western Iraq and eastern Syria. Here the U.S.-led coalition is making progress and has rolled back significant portions of the territory held by the terrorist group. But the gains have come from predominantly Kurdish and Shiite forces, and there are limits to how far these groups can advance into Sunni heartland areas and be accepted by local populations. Rolling back the Islamic State is not enough — to sustain these gains, we must focus on the security forces and governance mechanisms that will replace them.

The second theater lies farther west, where Syria is embroiled in a horrendous civil war. The United States has assumed that this problem is not as important and has heretofore avoided involvement except for pursuing diplomatic negotiations. That’s a mistake. In Syria and Iraq, the challenge of countering the Islamic State is bound up in the broader civil wars that have created governance and security vacuums and allowed the group to thrive. These vacuums are the disease; the Islamic State is the most serious of many problematic symptoms.

We propose a strategy that applies a consistent, long-term approach to Iraq and Syria, based on four interlocking efforts.

First, the United States should increase its support to those local armed groups that are acceptable to U.S. interests, whether they are fighting Syrian dictator Bashar al-Assad or the Islamic State. There is no central authority in Syria or Iraq that can seize, hold and govern all of the territories held by the Islamic State or the al-Qaeda affiliate Jabhat al-Nusra. Supporting more moderate local forces is the only way to secure these areas.

When we have used this approach in the past, it has brought success, from the Sunni Awakening during the surge in Iraq to recent advances by Kurdish and Sunni forces in northeast Syria. The groups we work with must be acceptable to local populations, which means supporting different groups in different areas, building security from the bottom up. In providing support to these groups, the United States must take steps to ensure that any weapons we provide them do not fall into the wrong hands.

Second, we must increase our overall military support. This should include expanding strikes and raids to take out Islamic State leaders and gain intelligence that stops terrorist attacks outside Islamic State territory. We should embed more U.S. combat advisers with local partners and provide our partners with support enabling them to do more, such as greater direct U.S. and coalition support from the air while they conduct attacks on the ground and air transport to move people and equipment quickly on the battlefield.

We should also explore whether there are viable limited military options using missiles and other long-range weapons from greater distances that will deter the Assad regime and its Russian patrons from bombing civilians and moderate opposition groups without leading to a major military escalation. Importantly, U.S. troops should not seize and hold territory or engage in large-scale ground combat operations. That must be done by local forces.

Third, we must do more to get important external actors on board. In the near term, the greater U.S. commitment we propose will appeal to key partners, especially Turkey and Saudi Arabia; we can use an increased commitment as leverage to get better cooperation on whom they arm and how we can together control the borders. Over time, this increased U.S. investment should also expand our leverage with Russia and Iran. They will not surrender their core interests, but greater U.S. involvement should open more space for a negotiated outcome.

Finally, we should focus on governance and politics by providing support to local municipal structures that can provide services and outgovern the extremists. And in time, as more acceptable groups gain influence and territory and as external actors become more pliable, the international community may be able to bring these conflicts to an end through negotiated outcomes that are based on keeping Syria and Iraq whole, but with highly decentralized governance models.

At this point, there are no good or easy options, only better and worse ones. To reduce and, eventually, eliminate the ability of the Islamic State to carry out attacks against the United States and its allies, the U.S.-led coalition will have to destroy the proto-state in Iraq and Syria and — something too often overlooked — ensure that it is replaced with an acceptable and sustainable alternative. We believe our approach can be executed at a cost commensurate with our national interests and, most important, can keep the American people safe.