QUEBEC -- The leaders of Canada and the United States are locked in an ugly, escalating public dispute over trade barriers, tariffs and how they think they world should resolve its problems.

With U.S. President Donald Trump and Prime Minister Justin Trudeau at the centre of the dispute, the back and forth has intensified since just before the start of the month -- when the Americans imposed hefty steel and aluminum tariffs on Canada.

The situation only got messier from there, particularly in the leadup to, during and right after the G7 leaders' summit. The G7 meeting in La Malbaie, Que., which was hosted by Trudeau, marked Trump's first visit to Canada as president.

Here's a blow-by-blow rundown of recent public exchanges that have led to an unprecedented political clash between otherwise friendly neighbours:

Trump, after announcing the tariffs, sends message to Trudeau about NAFTA talks on May 31-- "Earlier today, this message was conveyed to Prime Minister Justin Trudeau of Canada: The United State (sic) will agree to a fair deal, or there will be no deal at all."

Trudeau fires back on May 31 over the tariffs being applied on the premise Canada poses a national security threat to the U.S.-- "Let's be clear: These tariffs are totally unacceptable... That Canada could be considered a national security threat to the United States is inconceivable."

On June 1, Trump sends the against what he says are Canada's unfair trade policies -- "Canada has treated our Agricultural business and Farmers very poorly for a very long period of time. Highly restrictive on Trade! They must open their markets and take down their trade barriers! They report a really high surplus on trade with us. Do Timber & Lumber in U.S.?"

Trudeau, on the June 3 episode of NBC's "Meet the Press," uses stronger words to characterize Trump's tariffs -- "The idea that Canadian steel that's in military vehicles in the United States, that makes your fighter jets is somehow now a threat ... the idea that we are somehow a national security threat to the United States is quite frankly insulting and unacceptable."

Trump mentions Canada again in a couple more tweets about trade

...Canada has all sorts of trade barriers on our Agricultural products. Not acceptable!" He also posted this tweet: "Farmers have not been doing well for 15 years. Mexico, Canada, China and others have treated them unfairly. By the time I finish trade talks, that will change. Big trade barriers against U.S. farmers, and other businesses, will finally be broken. Massive trade deficits no longer!"

Asked about the president's Twitter blasts, Trudeau says June 7 that he won't sink to that level-- "I've been firm, I've been clear, but I don't think descending into insults is right for the way Canada engages with the world."

On June 7 -- the eve of Trudeau's G7 summit -- -- "Prime Minister Trudeau is being so indignant, bringing up the relationship that the U.S. and Canada had over the many years and all sorts of other things ... but he doesn't bring up the fact that they charge us up to 300% on dairy -- hurting our Farmers, killing our Agriculture!" "Please tell Prime Minister Trudeau and President Macron that they are charging the U.S. massive tariffs and create non-monetary barriers. The EU trade surplus with the U.S. is $151 Billion, and Canada keeps our farmers and others out. Look forward to seeing them tomorrow."

For good measure, before his arrival in Quebec-- "Canada charges the U.S. a 270% tariff on Dairy Products! They didn't tell you that, did they? Not fair to our farmers!"

In a news conference June 9 shortly before his departure from Quebec, Trump brings up his problems with trading with his friends -- "It's going to change, a hundred per cent. And tariffs are going to come way down, because people cannot continue to do that. We're like the piggybank that everybody is robbing. And that ends."

Later on June 9, after he left the G7 summit, likely while he was aboard Air Force One on his way to Singapore for his summit with North Korean leader Kim Jong Un -- "Just left the @G7 Summit in beautiful Canada. Great meetings and relationships with the six Country Leaders especially since they know I cannot allow them to apply large Tariffs and strong barriers to... U.S.A. Trade. They fully understand where I am coming from. After many decades, fair and reciprocal Trade will happen!"

At his closing G7 news conference, Trudeau once again refers to the national-security premise behind the tariffs as "kind of insulting" and then explains Canada's retaliation -- "I have made it very clear to the president that it is not something we relish doing, but it is something that we absolutely will do because Canadians are polite, we're reasonable, but we also will not be pushed around."

In response to Trudeau's news conference, -- once again, he was likely aboard the presidential aircraft -- "Based on Justin's false statements at his news conference, and the fact that Canada is charging massive Tariffs to our U.S. farmers, workers and companies, I have instructed our U.S. Reps not to endorse the Communique as we look at Tariffs on automobiles flooding the U.S. Market!" "PM Justin Trudeau of Canada acted so meek and mild during our @G7 meetings only to give a news conference after I left saying that, 'US Tariffs were kind of insulting' and he 'will not be pushed around.' Very dishonest & weak. Our Tariffs are in response to his of 270% on dairy!"

Larry Kudlow, Trump's top economic adviser, goes further Sunday than the president in attacking Trudeau-- "He really kind of stabbed us in the back... He did a great disservice to the whole G7."

Later Sunday, Trump's trade adviser Peter Navarro piles on -- "There's a special place in hell for any foreign leader that engages in bad-faith diplomacy with President Donald J. Trump and then tries to stab him in the back on the way out the door."

Following the remarks by Kudlow and Navarro, Foreign Affairs Minister Chrystia Freeland is far less confrontational in an effort, perhaps, to dial things down -- "In terms of the approach that governments choose to take, Canada does not believe that ad-hominem attacks are a particularly appropriate or useful way to conduct our relations with other countries."

suggesting that Canada is "bragging" in an unspecified release about benefiting from U.S. trade -- "Fair Trade is now to be called Fool Trade if it is not Reciprocal."