While the Trump administration has characterized their rift as a family quarrel, his fellow leaders appear to be itching for a fight with a USA president - the G7's traditional patriarch - during the two-day summit in the Canadian countryside.
Even Trudeau and French President Emmanuel Macron - who have both previously shown a willingness to swallow their differences with Trump - have voiced their willingness to cut the United States adrift if they can not reach some kind of consensus at the summit.
However, Mr Trump's regular tirades over the balance of trade between Japan and the USA has unsettled Tokyo and his apparent determination to engage with North Korea's leader Kim Jong Un has also caused some alarm in Japan.
But Mr Trump's imposition of tariffs on Canadian steel and aluminium imports on national security grounds infuriated Mr Trudeau who branded them an insult to the memories of Canadian soldiers who had died alongside American brothers in arms.
"I'll be very happy to be - in the name of all Quebecers - in Montreal today to greet [Macron] and also have a very important conversation with him on many, many accounts: mainly global trade, metal taridds that were introduced wrongly", Couillard said.
As tempers frayed, Trump had a ready retort, via tweet: "Please tell Prime Minister Trudeau and President Macron that they are charging the US massive tariffs and create non-monetary barriers".
After cautioning the U.S. leader during a media conference in Canada with the summit's host, Prime Minister Justin Trudeau, Mr Macron took to Twitter in English to insist the G7 could reach deals without Trump. "Look forward to seeing them tomorrow".
Trump may well be distracted by preparations for his June 12 summit with North Korea's Kim Jong Un, which will be in Singapore immediately after the rich-world talking shop in Canada. She was not named among world leaders due to sit with the U.S. president. Macron, who appeared to have built a warm relationship, said the "G6" leaders would not spark a fight at the summit.
"Until we can have reciprocal relationships, we will not have free trade, and we will not have fair trade", Kudlow said. The steel tariffs also effect the other G7 member, Japan, which was never granted a waiver.
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The Canadian leader is embroiled in rows with Washington over steel and aluminum and negotiations over the North American Free Trade Agreement. "The six countries of the G7 without the United States, are a bigger market taken together than the American market". The European Union has said that it's maintaining low expectations for the outcome. Mr Trump's singling out of Germany's auto sector for criticism in his gripes about the U.S. trade deficit hardly helped matters either.
Europe faces renewed domestic economic and political challenges in addition to those posed by Trump's unilateralism.
Trudeau also spoke by phone Thursday with Giuseppe Conte, Italy's new prime minister, and British Prime Minister Theresa May.
Japan is also expected to take a less confrontational approach than its G7 peers, while still quietly pressing its case on trade. British Prime Minister Theresa May urged the European Union to stick to World Trade Organisation rules and to ensure that any response to the tariffs was proportionate.
Tension has been building over a year of policymaking that has distanced the US from traditional allies, including by Trump's decisions to withdraw the USA from the Paris climate accord and the Iran nuclear agreement.
The pair have enjoyed each other's company on the golf course and issues such as the Iran nuclear deal which have soured relations with Europe have had relatively little impact on ties between Tokyo and Washington.
Mexican Economy Minister Ildefonso Guajardo said this week that Mexico expects to import pork cuts from Europe to compensate for the decline from the United States.
This time, the greatest threat to the liberal world order will be inside the fence.