But there's also another set of countries on the list—those that had militaries and then gave them up. Four of them are in Latin America and the Caribbean, which, Stearns told me, is "not usually a war-prone region" in the sense of countries invading each other (though the region does have a long history of civil wars and military dictatorships). Individual demilitarization stories differ. Costa Rica, for example, abolished its military in 1948 at the initiative of then-President José Figueres Ferrer, who, as the Council on Hemispheric Affairs has pointed out, himself came to power "through an armed insurrection" and may have abolished the military "in order to avoid a future potential military coup against him." The armies of Grenada and Panama, meanwhile, were both abolished following U.S. invasions in 1983 and 1989, respectively.

The pope's divisions aside, there's also a good deal of debate about what constitutes a military. Costa Rica, for example, is in a dangerous neighborhood along a key drug-smuggling route, and it also has a long-standing border dispute with its northern neighbor Nicaragua. The journalist Robert Beckhusen has argued that Costa Rica's 70-man Special Intervention Unit, a commando team that in his words "trains to intercept narco traffickers, in addition to rescuing hostages and acting as a high-intensity counter-terrorist unit," is a "military force in all but name." After 7,000 U.S. servicemembers deployed to the country in 2010 to assist in counter-narcotics efforts, Bolivian President Evo Morales said Costa Rica did have an army: the United States military.

"There's no perfect measure" of not having a military, Stearns said. Virtually all the countries the CIA lists as having no formal militaries have some form of security service, and the degree to which they focus exclusively on internal threats varies. Iceland, for example, is a member of NATO and participates in international peacekeeping missions with what the CIA calls "a civilian-led Crisis Response Unit." As Stearns noted, however, "In my opinion, if you commit to avoid external violence, you've taken an interesting step."

It's also potentially a cost-effective step. According to the CIA, military expenditures vacuumed up 2.42 percent of global GDP—some $2 trillion—in 2012. If you don't have to pay for an army, why would you?