"If North Korea would be ready to attack the United States," Prime Minister Stephen Harper said in 2006, "that would be a risk for Canada's national security as well not only because of our common values, but because of our geographical proximity." Much has happened in the intervening years apparently enough, if media reports are accurate, to force Canada to revisit its noncommittal position on missile defence. The case for participating in missile defence can be boiled down to four words: threats, technology, allies and cost.

Three decades ago, there were nine countries that fielded ballistic missiles. Today, there are 31. Several of them are unstable or unfriendly. But North Korea and Iran are the most worrisome.

The U.S. Defense Intelligence Agency concludes "with moderate confidence" that North Korea currently has nuclear weapons capable of delivery by ballistic missiles. Since 2009, North Korea has detonated two nuclear weapons, threatened nuclear strikes on the U.S., and demonstrated a threshold ICBM capability by lofting a satellite into orbit. In other words, North Korea's rockets aren't just a regional problem.

The Pentagon reported in 2012 that "Iran may be technically capable of flight-testing an intercontinental ballistic missile by 2015" a development that would bring North America within reach. The U.S. Missile Defense Agency (MDA) concludes that Iran has a missile capable of striking targets in Europe. And the British government revealed in 2011 that Iran has "been carrying out covert ballistic missile tests and rocket launches, including testing missiles capable of delivering a nuclear payload."

The good news amidst these worrisome developments is that missile defence is no longer just a theoretical possibility.

In testing, missile defence has scored successes on 59 of 74 attempts. During a 2012 exercise, the system deflected four out of five “near-simultaneous representative threats,” as MDA Director Vice Admiral James Syring reported in May. And in battle, missile-defence systems have protected population centers in Israel and military facilities in Kuwait from attack.

To their credit, those who have signed on to the international missile-defence coalition didn't delay participation until the system could guarantee 100 percent success an impossible standard.

Given Canada's historic contributions to allied efforts "from Normandy's beaches to NATO's founding, from Korea and Kuwait to Afghanistan and Libya" it's jarring to scan MDA's list of international partners and not see Canada's name.

The operative word here is "international." The missile shield now taking shape is a global network of networks enfolding some of Canada's closest allies and oldest friends.

In Europe, NATO leaders call missile defence "a core element of our collective defence." Britain and Denmark have allowed modifications to early-warning radars to augment the missile shield. Spain hosts a rotation of four U.S. Aegis missile-defence warships. Germany hosts a missile-defence operations center. Romania and Poland will host a land-based variant of the Aegis system, dubbed "Aegis Ashore," in the coming years. Turkey hosts a powerful X-Band missile-defence radar.

Beyond Europe, U.S.-Israeli cooperation has yielded a layered defence against short-range rockets and longer-range missiles. Israel and Qatar each host X-Band radars. And the UAE recently became the first foreign government to purchase a terminal high altitude air defence system (THAAD) from the U.S.

In the Asia-Pacific region, Australia and the U.S. signed a 25-year pact on missile-defence cooperation in 2004. Japan deploys six Aegis ships; hosts an X-Band radar, with another on the way; and is co-developing a new interceptor missile for Aegis ships.

Here in North America, the U.S. has deployed 30 ground-based interceptor missiles in California and Alaska, with 14 more on the way; two THAAD batteries, with more scheduled to come online; and 26 ships equipped with Aegis missile defences, building toward a fleet of 36.

Noting that "All the risks and nearly all the costs to build missile defences are borne by the Americans," a 2005 Fraser Institute study concluded that the costs of participating "are low and the benefits are high" for Canada.

Indeed, thanks to U.S. investments dating back to 1985, NATO has been able "plug into" the existing missile-defence  architecture for a relatively small amount, as NATO Secretary General Anders Fogh Rasmussen has explained, enabling the system "to defend European populations and territory."

Yet those who view government's main role as providing services might argue that spending on such big-ticket defence items diverts resources from social programs, while the market-minded among us might argue that such spending diverts resources from the private sector.

An answer to this conundrum comes from no less an authority on economic behavior than Adam Smith, who observed, "The first duty of the sovereign, that of protecting the society from the violence and invasion of other independent societies, can be performed only by means of a military force." He also noted that protecting society "grows gradually more and more expensive as the society advances."

This is not to suggest that there should be no limit on the amount devoted to defence, but rather to underscore that national defence is an essential duty of government—and that in an age of missile-armed rogues, it may require missile defences.