While Trump hailed the revised trade pact, Trudeau was more measured and used the event to call on Trump to remove the steel and aluminium tariffs the USA slapped on Canada and Mexico.
Mexican President Enrique Pena Neto, US President Donald Trump, and Canadian Prime Minister Justin Trudeau participate in the USMCA signing ceremony ahead of the G20 summit in Buenos Aires, Argentina.
"With the signing of this agreement, President Trump has delivered on his promise to renegotiate NAFTA and protect American farmers, ranchers, businesses, and workers", it said in a statement.
The vast majority of the pact still needs to be ratified by lawmakers in the three countries but the signing enacts a handful of immediate protections, such as from auto tariffs.
Maine's dairy industry could benefit from more market access in Canada, even though it will not be shipping milk across the border directly, said Julie Marie Bickford, from the Maine Dairy Industry Association.
Trump did not mention the Russian Federation investigation during brief comments before sitting down with Macri, whose father, the president noted, was a former business partner.
The USMCA emerged in early October, months after Trump hit Mexico and Canada with tariffs on their steel and aluminum products.
"This has been a battle, and battles sometimes make great friendships, so it's really terrific", Trump said of the combative process to overhaul the pact.
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NAFTA was signed by Canada, Mexico, and the United States in 1993 and came into force in 1994, during the presidency of Bill Clinton.
But Philip Levy, senior fellow at the Chicago Council on Global Affairs and a trade official in Republican President George W. Bush's White House, says: "President Trump has seriously overhyped this agreement".
While certainly a large trade deal, the USMCA is smaller than the Trans-Pacific Partnership was before Trump pulled out of it.
Mr Pena Nieto, who will be replaced on Saturday with Mexico's newly-elected president, has sold the deal to his people as a victory, saving free trade from a USA president who wanted to see it killed.
But it still faces an uphill battle in U.S. Congress, with some top Democrats in the House and Senate saying they are not convinced the new agreement is good enough for America. "We've taken a lot of barbs and a little abuse and we got there", Trump said.
Friday marked an important deadline for the trade pact because a new Mexican president takes over Saturday who might not honour the tentative deal struck by his predecessor. "We think this is simply wrong and frankly, we are very confident we are going to win those legal challenges". Trump tweeted before leaving his hotel for his first meeting, a short one with Argentinian President Mauricio Macri at Casa Rosada, the famous pink governmental palace.
For the record, the document's preamble refers to an "Agreement between the United States of America, the United Mexican States and Canada".
"However, we remain concerned that the continued imposition of steel and aluminium tariffs on Canada and Mexico will undermine the benefits of the USMCA", added Blunt, who heads the American Automotive Policy Council.
Trump opened Friday with a cordial meeting at the Casa Rosada with Argentine President Mauricio Macri, a longtime business acquaintance.