Carl Levin is a Democratic senator from Michigan. Angus King is an independent senator from Maine.

As members of the Senate Armed Services and intelligence committees, we knew much about the horrors in Syria before our recent visit to the region. But reading briefing slides in Washington is very different from meeting some of the hundreds of thousands of civilians who have fled for their lives to camps across the border.

They told us heartbreaking stories of how they sought shelter in mosques and schools, how Bashar al-Assad’s government targeted civilian neighborhoods for destruction, how their children still quake at harmless noises that remind them of the violence they sought to escape.

We believe the United States should join with its partners and allies in the region and elsewhere to pursue an end to the bloodshed. An international coalition that strengthens the military and political capabilities of thoroughly vetted anti-Assad forces should supply equipment and training. That coalition should also plan for steps that would place even greater military pressure on the Assad regime, including possible strikes against the missiles, aircraft and other heavy weapons that are the instruments of Assad’s campaign of terror.

These steps offer the best hope of convincing Assad and his few allies that a political solution is their only viable course.

No one — not our allies, not the Syrian opposition and not either of us — is advocating U.S. boots on the ground in Syria. That would not help the opposition and is unacceptable to the American people.

We do not underestimate the challenges to U.S. involvement in this crisis. The presence of anti-Assad forces that are aligned with al-Qaeda and other extremist groups means we must exercise great caution in whom we train and support.

But as substantial as these risks are, the costs of inaction are equally high. Assad’s survival, with support from Iran and Hezbollah, would surely strengthen them, to our great detriment. If Assad breaches the international consensus against the use of chemical weapons without repercussions, the United States — and every other nation — will be less secure. The intensifying violence in Syria has greatly stressed its neighbors and raised the specter of increased tension between Kurdish populations and governments in Turkey and Iraq.

In addition to these practical costs is the human price of inaction. If Assad’s campaign continues, he will kill more innocents, shell more homes into rubble and cause more innocent men, women and children to suffer. Stronger action in Syria is in our national security interests and is in keeping with our values.

Present in our minds, and in those of many other Americans, are bitter memories of the Iraq war, which we both opposed at the time. To be clear, we are reluctant to become, in any way, involved in yet another war in the Middle East. But U.S. national interests are at stake if the Syrian conflict continues to threaten regional stability, including that of partners such as Israel, Turkey and Jordan. It is critical to learn from our experiences in Iraq by taking a different approach to the conflict in Syria.

First and foremost, we propose that any action be taken in concert with our allies and international partners, in particular with other countries in the region and including Syria’s Arab and Muslim neighbors. Any strikes against the instruments of Assad’s terror should result from coalition deliberations — and we have called on our government and other members of the “London 11” nations to convene as soon as possible to agree to a comprehensive strategy to heighten the pressure on Assad. In contrast, the war in Iraq was launched with very little international support and a small military coalition that did not include a single Arab or Muslim country.

The situation is also far different in Syria, where a widespread insurgency has strong popular support. There was no similar broadly popular insurgency calling for our help in Iraq when the United States invaded in 2003.

And whereas Iraq involved a massive ground invasion by tens of thousands of U.S. troops, we oppose putting U.S. troops on the ground in Syria. We advocate careful planning as part of a broad coalition to support Syrians’ efforts to do what only they can do: force Assad and his sponsors to the negotiating table, where a transition plan can be developed to avoid a follow-on civil war and achieve regional stability.

Although there are no perfect historical parallels, the example to keep in mind today is not Iraq in 2003 but the Balkans in the 1990s, where limited U.S. involvement, in close concert with an international coalition and in support of indigenous forces, helped shape the conditions for a negotiated settlement that ended ethnic cleansing and protected important U.S. national security interests. We can, and should, help the Syrian people end the senseless slaughter they are suffering in a manner consistent with our interests and values.