France and Europe are standing by the G7 communique, a French presidency official said, adding anyone departing from the commitments made at the summit would be showing their "incoherence and inconsistency".
In his news conference, Mr Trudeau had spoken of retaliatory measures that Canada would take next month in response to Mr Trump's decision to slap tariffs on steel and aluminium imports. "We joined the communique in good faith", Kudlow said.
Mulroney - who was prime minister when both the original Canada-U.S.
Even Hollywood actor Robert de Niro was weighing in.
And on Twitter, the hashtag #ThanksCanada was trending with Americans thanking their neighbours to the north for everything from Nanaimo bars to hockey to caring for airline passengers during 9/11.
The group managed a common statement in which they agreed to disagree on some issues, only to have Trump disavow the document in a tweet after leaving the meeting. "Least of all from subordinates of the president directed at the prime minister of their greatest friend and ally", he said.
Trump's comments after a historic summit in Singapore with North Korean leader Kim Jong Un were the president's most extensive on the matter since he tweeted that Trudeau was "very dishonest and weak" and raised the prospect of tariffs against auto imports, a move that would imperil the Canadian economy.
In view of threatened United States tariffs on vehicle imports, which would particularly affect Germany, Grenell said that he was a "big auto fan" and knew how important the industry was. "And that's what bad faith Justin Trudeau did with that stunt press conference".
And while she joined with Mr. Trudeau and other European leaders in opposing the USA decision to slap import tariffs on steel and aluminum, she urged continued dialogue.
Trump threatened that the U.S. will stop trading with countries that maintain tariffs on his country's exports, particularly in the agricultural sector, and that he has launched the idea of eliminating all trade duties, because "ultimately that's what you want".
The tariffs, as well as Trump's noted opposition to multilateralism, his criticism of Canadian trade barriers and suggestion that the union become the G8 with the inclusion of Russian Federation made for a chaotic lead into the G7 summit.
And Roland Paris, a former foreign policy adviser to Trudeau, jabbed at Trump on Twitter: "Big tough guy once he's back on his airplane".
"International cooperation can not depend on being angry and on sound bites".
Mr Trump also responded by announcing that he was backing out of a joint communiqué on trade between the United States and its main allies. Edward Markey (D-Mass.) said on CBS' "Face The Nation" that "right now, the President is actually driving our allies away from us as we need them even more, while welcoming in the Russians".