The best way for Canada to defend its people from a nuclear-armed North Korea is to join the  U.S. anti-ballistic missile defence program, argues a former chief of the defence staff.

And Tom Lawson, who served as Canada's top general under former prime minister Stephen Harper, said he's heartened to see signals from the Liberal government that they might be open to it.

"If a rogue nation fired a ballistic missile at the United States and it went off course towards Canada, it's not a certain thing that that missile would be brought down by American ballistic missile defence, because Canada hasn't signed on," said Lawson, who also served as the deputy commander of the North American Aerospace Defence Command.

North Korean missile launch, July 4

This picture taken on July 4, 2017 and released by North Korea's official Korean Central News Agency (KCNA) on July 5, 2017 shows the successful test-fire of the intercontinental ballistic missile Hwasong-14 at an undisclosed location. (STR/AFP/Getty Images)

"It just really makes sense that if the Americans are asking Canadians to come on board this would provide Canadian politicians, at times of crises, a say on what happens with that portion of Norad defence."

Lawson said just a few years ago it looked more unlikely that North Korea would be able to build up its nuclear capacity to what it has today. North Korea's leader, Kim Jong-un, now claims his country has missiles with the range to hit Chicago.

'Emerging threats' covered in defence plan

The Liberal government chose in its recent defence policy to remain outside of the U.S. anti-ballistic missile program, upholding a decision made more than a decade ago by former prime minister Paul Martin.

But Lawson said the defence policy does have a line about engaging "the United States to look more broadly at emerging threats and perils to North America, across all domains as part of Norad modernization."

"What it means is we've got an open mind to other things," said Lawson.

"We should come on board. There is kind of an open invitation from our American hosts […] Most military advisers find it a very odd stance that Canadian politicians have taken, that we don't get on board with ballistic missile defence."

Former defence minister Peter MacKay said recently he laments not joining the U.S. ballistic missile defence program when he had the chance.

The policy was a matter of intense debate for the former Conservative government, as it was for the Liberals.

"We share a great deal of intelligence with the United States and if they're alarmed, we should be alarmed," MacKay told CBC News last month.

Not being involved in the program is "a huge problem," he added.