The above map shows the embassies, consulates, etc. affected by the closure order.

On Friday, President Obama closed two dozen embassies, consulates, and other diplomatic outposts, and on Sunday, extended the closures of 19 of those through the rest of this week. The move won praise on both sides of the aisle as a smart response to a major threat from al-Qaeda, what Sen. Saxby Chambliss (R-GA) called, "the most serious threat that I’ve seen in the last several years." The administration also responded by closing roads near the facilities, erecting blast walls, and taking other precautionary measures.

Here's the problem - that stuff never works.

It's easy to see the temptation to close the embassies and boost their security, as this wouldn't be the first time a U.S. embassy was attacked by al-Qaeda. 1998's attacks on the Dar es Salaam and Nairobi embassies killed 223 people and injured over 4,000. But researchers who've looked into the question have found that securing embassies in response to terrorist threats is, at best, ineffective, and at worst has the perverse effect of increasing casualties.

Cynthia Lum at George Mason and Leslie Kennedy and Alison Sherley at Rutgers conducted a large meta-analysis of studies evaluating the effectiveness of counterterrorism tactics, including embassy fortification, in 2006. They analyzed a total of 28 findings from four papers, two by Walter Enders (at the University of Alabama) and Todd Sandler (at University of Texas - Dallas), one by Enders, Sandler, and Jon Cauley (at the University of Hawaii - Hilo), and one by Cauley and Eric Iksoon Im (also at Hilo).

Just so we're clear on what we're talking about, here's what the 1993 Enders and Sandler paper looked at:

In 1976, spending on embassy security  more than doubled. Significant additional resources  were allocated to security in the 1980s as a result of  the takeover of the U.S. embassy in Tehran on 4  November 1979. Security measures included the installation of metal detectors to screen visitors to  embassies. Large increases in security were authorized to start on October 1985 by Public Law 98-533.  On 12 August 1986, the U.S. Congress passed legislation authorizing and additional $2.4 billion over five years to rebuild and fortify U.S. embassies and missions.

In 19, there was no significant effect of increased embassy security. In three, increased security was helpful, and in six, it actually was harmful. This chart from the evaluation summarizes the research well:

"In total, the findings do not indicate that the fortification of embassies and efforts to protect diplomats have been effective in reducing terrorist attacks on these targets," Lum, Kennedy, and Sherley conclude. Worse, "there does not appear any logical grouping of the types of outcomes in which harmful or beneficial outcomes occur." So not only do the majority of findings suggest that fortifying embassies doesn't work; there's nothing connecting the minority of cases where it does.

How could this be? Mostly, there's a substitution effect. Enders and Sandler in 1993 found that securing embassies reduced attacks on U.S. interests, but increased assassinations, hostage-takings, and airplane hijackings. The assassinations and the other incidents just happened when embassy personnel weren't in protected areas.

Does this mean the latest State Department effort is pointless? Not necessarily; perhaps we've gotten smarter at securing facilities since the events analyzed in these studies. But it does suggest that we shouldn't be too optimistic about what embassy security measures can do.