The G20 accounts for two-thirds of the world population and is widely credited with avoiding a major global calamity after the 2008 financial crisis.
In the two minutes of the Buenos Aires meeting open to the media, Mr Trump's first remarks suggested he was still coming to grips with the Australian leadership change that had now delivered the second prime minister of his presidency.
"Now, this task is even more urgent" because trade protectionist measures are on the rise and the 2018 global commodity trade turnover is expected to shrink, he said. "We call on all members to oppose such WTO-inconsistent measures, stand by their commitments undertaken in the WTO". Trump has threatened fresh tariffs and set a January deadline for raising the existing ones from 10% to 25% on a range of goods.
"In terms of economic effects, Chinese export growth will likely weaken in coming months on payback from recent "front-loading", and - unless a deal is reached - over time by some USA substitution to non-Chinese suppliers", Goldman Sachs wrote in a note.
For the wider G20 meeting, consensus is proving elusive.
The G20 summit is the biggest global gathering in Argentina's history, and the protest underlined security concerns after recent football violence forced the relocation of a showpiece final to Spain. In previous years, the G20's joint statement had been hammered out well in advance.
"That this deal sets a path for the United Kingdom to a brighter future has been affirmed by the discussions I've had on trade over the past two days, with friends and partners making clear that they are keen to sign and implement ambitious free trade agreements with us as soon as possible", she said.
The meeting with Xi comes after a victory for Mr. Trump on trade.
In Buenos Aires, Trump is also due to sit down with Putin for a bilateral meeting that has taken on added import after Russian Federation fired on and detained three Ukrainian ships in the Sea of Azov near Crimea.
The two leaders made headlines nonetheless through a hearty handshake ahead of the G20's family photo event.
Navarro's presence at the dinner table alongside Trump reflects a change in plans after the White House said Navarro wouldn't join Trump's more moderate advisers like Treasury Secretary Steven Mnuchin for the trip to Argentina.
Trump himself said Bush had "inspired generations of his fellow Americans to public service".
Trump was also scheduled to speak with German Chancellor Angela Merkel and Turkish President Recep Tayyip Erdogan.
He did not respond to a reporter's question about whether he regrets his past criticism of the 41st president, and his son, George W. Bush, who was the 43rd president.
A White House official said that Trump sees a "good possibility" for a deal to defuse the trade frictions, as Washington demands greater market access and protections for intellectual property by Beijing. "And we´ll see", the president said.
Trump's G20 summit was overshadowed by his abrupt decision to cancel a meeting with Russian President Vladimir Putin.
Both sides have certainly come prepared for serious discussion.
As soon as he arrived, the crown prince was confronted by French President Emmanuel Macron, who pressed him on the Khashoggi investigation and the Saudi-backed war in Yemen. Putin reportedly pulled out a pen and paper to sketch the skirmish in the meeting, Bloomberg reported.
Growing concerns that the trade war will increasingly hurt corporate earnings and the US economy are a key reason why USA stock prices have been sinking this fall.
New export orders declined for a sixth straight month, according to an official survey of Chinese manufacturers.