As Fourth of July celebrations take place across the United States, they inadvertently will provide terrorists with potential so-called “soft targets.”

Soft targets are more vulnerable to terrorism because they have lower levels of security, making them easier to attack, whether by a lone actor or a small team of terrorists relying on basic surveillance and simple plans. The past several years have shown that no community is totally immune from this threat. Any public event can easily be a target—a club in Orlando, an office holiday party in San Bernardino, or a marathon in Boston.

Attacks that kill a significant number of people (such as the forty-nine who died more than a year ago in Orlando) or are directed against a symbolic target, meaning it has special significance to a city or to the country (the 2013 Boston Marathon bombing), will likely result in increased media coverage, a primary goal of most terrorist groups.

In the past year, terrorists inspired by the Islamic State have been opportunistic, increasingly tailoring their attacks to relatively simple targets by using vehicles as ramming devices. And while New Jersey–style metal barriers and stanchions can help mitigate this threat, it is impossible to secure all public spaces from vehicular terrorism.

The sheer range of soft targets seems endless—and include shopping malls, sports arenas, museums, restaurants, airports, concert venues and public transportation systems. And not all soft targets are created equal. Because of heightened security measures, it would be more difficult for terrorists to attack a sports stadium or public-transit hub, for instance, than a shopping mall or restaurant.

In other parts of the world, terrorists have targeted hotel chains based in Western countries or those frequented by westerners. Between 2002 and 2015, these hotels have been attacked in Pakistan, Indonesia, Egypt, Jordan and Mali. These attacks, some of which included suicide bombings, resulted in many deaths and widespread property damage, while also having a negative impact on Western tourism. Terrorism expert Peter Bergen said he believes that these kinds of hotels are attractive terrorist targets for two primary reasons: Since they are in the hospitality business, they are unlikely to “turn themselves into fortresses” and they are likely to be frequented by Western tourists, an attractive target.

In the United States, terrorists have frequently targeted transportation, from softer targets like commuter trains and subways to more hardened targets like commercial aviation. The commandeering of airplanes on Sept. 11, 2001, to attack buildings in New York City and Washington, DC is the most high-profile example of the latter. And while American authorities worked assiduously to increase defensive measures designed to protect transportation infrastructure, these types of targets continue to be attractive to terrorists.

After receiving training in Pakistan in 2007, Al Qaeda recruit Bryant Neal Vinas plotted to bomb the Long Island Rail Road, a busy commuter-train service that connects New York City to its outer suburbs. In 2009, Afghan immigrant Najibullah Zazi was arrested before he could carry out his plan to bomb the New York City subway system. Commuter trains and buses have been successfully attacked by terrorists in Madrid, London and Moscow.

Meanwhile, the focus on airplanes remains a high priority for terrorists linked to Al Qaeda and the Islamic State. In June, the New York Times reported that Israeli intelligence had gleaned from ISIS operatives information about the group’s quest to make explosives that look exactly like laptop computer batteries in an effort to fool X-ray machines at airports.

As these examples show, soft targets chosen by terrorists share three important characteristics—they are frequented by many people, are easily accessible and can be surveilled without drawing too much attention to the potential attackers. Oftentimes, these targets can be monitored virtually through imagery and details available online. As a whole, soft targets are hard to defend—there are too many of them and defense would require an enormous application of resources.

The United States is forced to make choices regarding where to apply limited resources to the defense of soft targets. Providing better security for federal, state and local government offices and agencies as well as transportation hubs like airports and train stations has, rightly, been the focus of these efforts. However, when one potential target is hardened, other softer targets become more susceptible to attack.

So what, if anything, can the United States do to protect soft targets from terrorist attacks?

The U.S. government should redouble its information-sharing efforts with other governments. It should also expand programs already in place for sharing information with local law enforcement and provide increased resources for sustainment, training and other initiatives dedicated to this kind of threat. Many terrorists or would-be terrorists are increasingly referred to as “known wolves” because they are known to intelligence and police agencies prior to an attack. Broad sharing of information among intelligence agencies and police services coupled with additional training could help identify and track potential attackers before they can execute their plans.

State and local governments should also consider establishing where appropriate heightened security standards—not just for frequently traveled public spaces but also for privately owned businesses and venues. U.S. government agencies like the Department of Homeland Security already provide additional resources for high-profile events like the Super Bowl and on balance, have done an admirable job of providing security without being overly invasive.

Better public-private partnerships are necessary to protect soft targets from attack and to ensure that first responders and emergency medical services are included in planning from the outset. The relationship between the public and private sectors should be mutually beneficial. After all, reliable and consistent security is good for business.

While many businesses already implement a variety of physical security features based on their specific needs—closed-circuit television, perimeter fencing and lighting, automatic door-locking after hours—the government should continue to work with them to define thresholds where additional security assets and procedures are not optional but required. Enhanced standards should be set for lighting, guards, identification and other preventive measures for occasions when hotels, concert halls, or convention sites are hosting events or are expecting large crowds. Furthermore, businesses should be required to coordinate with applicable law-enforcement agencies during and prior to events where larger-than-normal crowds are expected.

In addition, public-information campaigns like “see something, say something” should be advertised prominently in public spaces prior to or during events where large crowds are expected to gather. Because police and guards cannot be deployed everywhere or in sufficient numbers to secure everything and everyone, the first line of defense against a terrorist attack is oftentimes a vigilant and aware population.

Colin P. Clarke and Chad C. Serena are political scientists at the nonprofit, nonpartisan RAND Corporation.

Image: A couple (L) and a jogger pass by the Lincoln Memorial Reflecting Pool before sunrise in Washington, September 24, 2013. The nation's capital is enjoying mild temperatures during the first week of autumn. REUTERS/Jason Reed.