Frank McKenna, the former Canadian ambassador to the United States, says the federal Liberals should support more joint security initiatives with the United States – including rethinking their decision not to join the American missile defence program.

In early 2005, then prime minister Paul Martin and his Liberal government rejected Canadian participation in the program. But in a paper released on Monday, McKenna argues that it's time for the party to rethink that decision.

"It certainly would be timely in view of the escalating threat around the world for us to have this dialogue," McKenna told CBC News.

He wrote the discussion paper on Canada-U.S. relations earlier in the year for the federal Liberal party's renewal committee.

Trading relationship at stake, McKenna warns

"We must never forget that another 9/11, or a major event of a similar nature that resulted from a laxness with respect to Canadian security, would have a profound impact on trade between the two countries and, consequently, our national wealth," McKenna writes.

The world's largest trading relationship is at stake if Canada feeds the perception that it is not a safe neighbour, his paper argues.

McKenna was New Brunswick's premier from 1987 to 1997 and served as ambassador to the United States from March 2005 to January 2006. He resigned soon after the Conservatives took power and they later appointed former finance minister Michael Wilson.

McKenna was considered a top candidate for Liberal leadership but decided not to run.

Prime Minister Stephen Harper has not said what Canada's role will be in the U.S. missile defence program. In September, the Senate defence committee recommended that Canada should join the ballistic missile defence program, criticizing the decision by the former Liberal government to opt out.

When Martin announced his position in February 2005, he also said: "Let me be clear: we respect the right of the United States to defend itself and its people."

His comments came on the heels of McKenna causing a controversy in Ottawa by stating Canada was already taking part in the program because it had agreed the North American Aerospace Defence Command could monitor the skies for incoming missiles.

"There's no doubt, in looking back, that the Norad amendment has given, has created part — in fact a great deal — of what the United States means in terms of being able to get the input for defensive weaponry," said McKenna at the time.

'Americans … just never understood our position'

McKenna told CBC News that U.S. lawmakers he talked to during his time as ambassador were disappointed with Canada's decision not to join in the war in Iraq but understood it to a certain degree. That wasn't the case with the missile defence issue, he said.

"Americans on this one just never understood our position," he said. "They thought it was poorly communicated and they couldn't understand why, when they weren't asking for anything from us, why we were so hostile to participating even in a dialogue about the defence of North America, including Canada."

In his paper, McKenna says Canada must also be prepared to address U.S. security concerns by adopting the highest possible standards of technology, identification documents and an immigration system that can withstand scrutiny.

"The exercise of sovereignty for Canada should not be an emotional exercise that colours every issue we have vis-a-vis the United States," he said. "I don't think our sovereignty is in peril."

With files from the Canadian Press