One of the biggest threats to American security is not the beleaguered refugee program, which has admitted barely 2,000 Syrians over the last several years.

No, the soft underbelly of America's security system is one that Sen. Dianne Feinstein, D-Calif., calls "the Achilles' heel of America" — the Visa Waiver Program. Little known outside those who frequently travel abroad, this program lets 20 million foreign visitors a year into the U.S. for up to three months without a visa.

Begun in the mid-1980s to facilitate business and tourism travel, the visa program covers 38 countries, including Belgium — home to the planner of the Paris attacks — and most of Europe, whose porous borders and subpar security measures have been in the spotlight. The U.S. can no longer afford to be so generous in its attempts to ease foreign travel. And, as Paris proves, deciding which countries are "safe" has become more difficult than ever.

National security experts estimate that up to 4,000 people now in Europe have traveled to Syria or Iraq to train with jihadists. They fear that some of those could attempt to enter the U.S. on a European passport through the waiver program without ever undergoing in-depth screening.

In the aftermath of the latest attack, some in Congress called for an immediate shutdown of the refugee program — where applicants undergo an exhaustive screening process that can take two years. But in the Senate, a bipartisan group led by Feinstein and Sen. Jeff Flake, R-Ariz., started working in earnest on the far greater vulnerabilities in the waiver program. That group included Sen. Amy Klobuchar, D-Minn.

The bill they've proposed includes sensible restrictions that would do more to keep out those who might do harm to the U.S. than any changes to the refugee program. Chief among them would be making anyone who had traveled to Syria and Iraq in the past five years ineligible for a waiver. Instead, such travelers would have to undergo a personal interview at an American Embassy and in-depth screening as part of the traditional visa process.

Travelers applying for waivers would have to provide an electronic passport and biometric information, including photo and fingerprints. "We want that information before they get on the plane, not after," Klobuchar said.

Already thousands of applicants to the waiver program have been found trying to enter on passports reported lost or stolen, or bear names that show up on no-fly lists. The proposed precautions in this bill are sensible steps to protect U.S. security that must be taken, despite the pushback that has begun by some interest groups. The U.S. Travel Association already has voiced concerns about the biometric requirements.

But after more than a decade of having to take off one's shoes to board an airplane, being screened at the skeletal level in U.S. airports and enduring sometimes invasive pat-downs, is taking a photo and fingerprints really an impediment to travel? Might it not prove far more effective in targeting those who pose an actual danger?

Klobuchar said momentum is growing in Congress to pass a bill before this year's recess. Once passed, Congress should next consider expanding the list of no-waiver countries beyond Syria and Iraq. ISIL now claims territory not just in those countries, but in Libya, Yemen, Algeria, Egypt, Saudi Arabia and Pakistan. Klobuchar said there has been "major discussion" of such an expansion.

If the U.S. hopes to preserve national security, it must focus its efforts where they will be most effective. Tightening the Visa Waiver Program is a good start.