Prime Minister Justin Trudeau and other world leaders are gearing up for what already promises to be a lively debate on defence spending at the upcoming NATO summit in Brussels next week.

On Tuesday, the Prime Minister's Office said Trudeau is looking forward to meeting with the 28 other NATO leaders to discuss ways to reinforce peace and security among nations.

"Throughout the summit's discussions and working sessions, the prime minister will reiterate Canada's commitment to playing an active role in the alliance, and a strong and constructive role in the world," said Trudeau's press secretary Eleanore Catenaro.

The PMO also said that just prior to the summit, Trudeau will visit Canadian troops in Latvia, where Canada is leading a key NATO battle group established as the alliance's response to Russia's surprise annexation of Crimea in 2014 and its invasion of eastern Ukraine.

The prospect of conflict among NATO leaders is already looming on the horizon, thanks to a series of pointed letters from U.S. President Donald Trump to leaders of several NATO allies, including Canada, calling on them to finally meet the alliance's defence spending targets.

In his letter to Trudeau, Trump says there is "growing frustration" in the U.S. with North Atlantic Treaty Organization allies like Canada that have not increased defence spending as promised.

"This frustration is not confined to our executive branch. The United States Congress has taken note and is concerned as well," the president writes in the June 19 letter.

"The United States is increasingly unwilling to ignore this alliance's failure to meet shared security challenges."

Meeting the NATO commitment

The letter comes with tensions between Canada and the U.S. running high, thanks to an ongoing dispute over American tariffs on steel and aluminum imports that have prompted Canada and other European countries to impose politically targeted retaliatory tariffs.

It also comes in the wake of a stormy end to the G7 meetings in Quebec, when Trump called Canada's prime minister "dishonest and weak" and backed out of the final joint communique issued by the G7 leaders after hearing Trudeau's defiant comments over the tariff dispute.

The Liberals promised last year to increase spending on the military by 70 per cent over the next 10 years, but Canada continues to fall short of NATO's target of spending two per cent of GDP on defence. In 2017, the alliance's own preliminary estimates showed Canada spent 1.29 per cent of its gross domestic product on defence, up from 1.16 per cent in 2016.

Trump is not the first American president to complain about NATO member countries not living up to their spending commitments and relying too heavily on the U.S., which last year spent 3.57 per cent of its GDP on defence.

Slow progress

Currently, only four countries that belong to NATO are meeting the two per cent target that allies agreed to during the 2014 summit in Wales.

Trump has threatened to leave NATO if member states do not follow through with their pledges.

But NATO Secretary General Jens Stoltenberg noted a marked improvement in defence spending among member nations since 2014 in his annual report, issued earlier this year.

Three consecutive years of growth in defence expenditure across Europe and Canada have added a total of $46 billion to defence, he said.

In 2017 European allies and Canada increased their defence expenditures by almost five per cent. This year, eight NATO countries are expected to meet the two per cent guideline while many others have plans to meet the target by 2024.

"So the picture is clear — the alliance is doing more to respond and adapt to an uncertain security environment," Stoltenberg says in the report. "All allies are stepping up, doing more, in more places, in more ways, to strengthen our shared security."

Trudeau in Latvia

That, however, isn't good enough for Trump.

Canada's continued defence spending of less than two per cent "provides validation for other allies that also are not meeting their defence spending commitments," he writes.

"I understand domestic political pressures, as I myself have expended considerable political capital to increase America's defence spending," Trump's letter continues.

"It will, however, become increasingly difficult to justify to American citizens why some countries continue to fail to meet our shared collective security agreements."

Defence Minister Harjit Sajjan's office pointed to Canada's commitment 10-year commitment to grow funding to the department, which will see $32.7 billion in annual increases.

"Also, our recent and ongoing contributions to NATO, such as our mission in Latvia, are clear demonstrations of our government's commitment to the alliance and international security ," Sajjan's director of communications Renee Filiatrault said in statement Tuesday.

Trudeau will visit Canadian troops in Latvia July 9-10 before attending the NATO summit July 11-12.