The Conservative government is believed to be facing a request by the United States to join an anti-ballistic missile shield.
The request is coming as the Americans ramp up their own protection in response to increased tension with North Korea and Iran.
In March, the Pentagon announced its intention to place 14 new ground-based missile interceptors in Alaska by 2017.
That suggests the U.S. sees a threat to their northern territory as a possibility, raising the question of Canada's exposure and also its responsibility.
A spokesman for Defence Minister Peter MacKay wouldn't discuss the request.
"Canada has declined to participate in ballistic missile defence in the past," Jay Paxton said.
"We constantly review the security situation internationally."
Public Safety Minister Vic Toews said Sunday that a conversation was necessary.
"I think we need to have a broader discussion about that and I'm not prepared to venture an opinion at this time,” he told CTV's Question Period.
"What I can say is co-operation with our allies, especially in relation to a terrorism-related threat, is absolutely essential to keeping Canadians safe."
What role Canada should play in continental missile defence has bedevilled many a former government.
The issue was responsible for the defeat of John Diefenbaker's Conservative government over a joint-missile program with the U.S. that included nuclear warheads.
During the Cold War, then prime minister Pierre Trudeau allowed the U.S. to test cruise missiles over Canadian soil, riling the public.
Liberal Prime Minister Paul Martin also faced massive pressure from the U.S. to join a missile shield program in 2005, ultimately saying no.
NDP Foreign Affairs Critic Paul Dewar noted that reason Canada said no in 2005 was the belief it wouldn't enhance security.
"What we really need to look at with North Korea is how to work particularly with the Chinese to force them to force the North Koreans to stand down," he told Question Period.
During the 2006 election, Harper said he'd be open to revisiting the issue.
"If the Americans propose such an arrangement, and if we come to the conclusion that it's in the country's best interests, it's my intention to turn this treaty over to Parliament for a free vote," he said at the time.
A year later, the Canadian co-chair of the U.S.-Canada Permanent Joint Board on Defence suggested the issue should be renewed.
The co-chair at the time was Tory MP Rick Casson.
"Co-Chairman Casson — who chairs the Standing Committee on National Defence in the Canadian House of Commons — also wondered aloud at the end of the meeting if Canada and the United States should revisit Continental Defence at the next meeting of the PJBD," reads a 2007 cable from the U.S. embassy in Ottawa posted by the website Wikileaks.
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