There's a natural question following yesterday's attack in Boston: What would it have taken to keep it from happening? It's a question that will be asked by law enforcement and politicians and the media over the next few weeks. And the answer is pretty simple: More than we'd likely want to accept in a free society.

In an email to Politico this morning, veteran journalist Tom Brokaw offered that "we're the most advanced nation in the world, living with Third World vulnerabilities." That's wrong. The vulnerabilities we live with are first world vulnerabilities: an emphasis on protecting individual liberty despite the need to maintain public safety. Security, we are reminded after events like this, is a balancing act.

Let's say, for the sake of argument, that some authority wanted to ensure that what happened in Boston couldn't happen again. The first question is: What happened in Boston? Two bomb sweeps preceded the race, meaning that the bombs were likely brought by an attendee. The marathon course was largely open to the public, save the final 100 meters of the race on the south side of Boylston Street which was reserved for race officials and invited guests. The explosions happened across from that zone, in a publicly accessible area. "[M]any fans arrive at the finish area in the early hours of the morning to secure the best viewing area," the race spectator guide reads.

Details about the explosive devices haven't been made public by the police, but there are reports that they were black-powder bombs, packed with ball bearings or other shrapnel. How the devices were hidden isn't clear, with some suggestions that they were placed in garbage cans at the site.

Various things have been proposed or implemented following similar events.

Removal of containers that could house bombs.

Following a series of bombings by the Irish Republican Army, the United Kingdom removed trash cans from train stations in the 1980s. If yesterday's bombs were hidden in trash cans, removing them may have made it harder for an attacker.

Likelihood of preventing a bombing: Low. Removing or sealing garbage cans in the UK did little to stem the tide of bombings. While it is not uncommon for the Secret Service to remove mailboxes and seal manholes prior to a visit from the President, this makes it more difficult to hide a bomb, not impossible. A bomb could be hidden in crevices in the facade of a building on a storefront, or in a tree, or in a restroom. There are various degrees of obviousness about these various proposals, of course; throwing a package in a garbage can is far less obvious than tucking it under a windowsill. But they are all also more obvious than putting a bomb in a backpack, setting it on the sidewalk, and asking someone nearby to keep an eye on it for a second.

Infringement on civil liberties: None, unless you count being able to easily throw away garbage a civil liberty issue.

Increased use of security cameras.

After the attempted 2010 car bombing of Times Square, proposals to increase security camera coverage were introduced. New York City Mayor Michael Bloomberg has embraced the idea of expanded surveillance, saying:

We're going to have more visibility and less privacy. I don't see how you stop that. And it's not a question of whether I think it's good or bad. I just don't see how you could stop that because we're going to have them.

Likelihood of preventing a bombing: It depends. There were certainly a lot of cameras trained on the finish line of the Boston Marathon yesterday. As we mentioned, two bomb sweeps preceded the race, indicating that the devices were most likely planted after spectators arrived. The cameras may very well aid in catching the culprit, but it's not clear that more cameras would have done much to prevent the attack. More cameras require more people manning them — and more cost.

Infringement on civil liberties: High. Shortly after 9/11, the ACLU wrote a post titled, "What's Wrong With Public Video Surveillance?" Their answer: CCTV is prone to abuse and usually lacks controls over when and how images can be used. The first point the article makes is this one: Video surveillance has not been proven effective, particularly for incidents of terror.

Restrictions on availability of bomb-making material.

Pointing to the Oklahoma City bombing sixteen years prior, the Department of Homeland Security in 2011 proposed limiting the availability of the fertilizer used in that attack. In this case, the targeted restriction would likely want to focus on black powder.

Likelihood of preventing a bombing: Low. Buying black powder in the state of Massachusetts requires a gun license and a firearms identification card. Banning it entirely would require legislation in the state that might raise Second Amendment concerns; black powder is used in certain firearms and for some ammunition. Not that a Massachusetts ban would be enough. Any state would need either a national ban on sales or production of the powder, or to have bomb dogs sniffing every car or person entering or leaving.

Infringement on civil liberties: Medium. Gun advocates would almost certainly argue that a restriction on black powder is an infringement on their Constitutional rights.

Increased security at vulnerable locations.

This was the strategy used by Homeland Security after 9/11: tightly securing access to airplanes and airports.

Likelihood of preventing a bombing: Medium. While the DHS changes at airports preceded a period during which there were no airplane hijackings and no domestic bombing attempts, it's not clear the extent to which those things are correlated.

But it's much easier to implement tighter restrictions at airports than it is to secure both sides of a 26-mile race course that winds through residential and commercial districts. The only area of the race course yesterday to which access was limited were the areas in gray below and the stretch along the south side of Boylston Street near the finish line. It was right across from that area that the first explosion occurred.

Infringement on civil liberties: Medium. While no one has to attend a sporting event, a large increase in searches to gain access to the event would be intrusive — if even possible.

Focus on identifying and stopping potential terrorists.

After the 9/11 attacks, both the FBI and the NYPD sent undercover officers into Muslim communities and mosques in order to ferret out any bomb plots. A number of plots have since been revealed.

Likelihood of preventing a bombing: Low. Many of the plots that the FBI has disrupted were ones in which the only dangerous elements were promised by undercover FBI agents. The 9/11 attacks also provided a ready profile of an attacker: a person of Muslim faith, often of Middle Eastern birth. In cases of domestic terror, which Boston may turn out to be, the profile has historically been much looser and much more broadly applicable to a broad swath of the American population.

Infringement on civil liberties: High. The FBI and NYPD have been repeatedly criticized for the way in which they've targeted Muslim-Americans and resident immigrants.