The G20 summit in Rio de Janeiro, which begins on Monday, is already impacted by ever-evolving geopolitical events, regardless of host Brazil's official theme of fighting hunger, poverty and inequality.

Prime Minister Justin Trudeau is in Brazil on the heels of the Asia Pacific Economic Co-operation (APEC) summit in Lima, Peru's capital, where he conceded that many conversations with other nations were about preparing for a Donald Trump presidency and about what can be accomplished before he takes over the White House.

"I think there probably are elements of ... accelerating certain things in the coming months for some," Trudeau said at his closing news conference at the APEC summit.

The G20 will likely be no different, especially in light of multiple media reports that U.S. President Joe Biden has decided to lift restrictions on Ukraine so that it can now use weapons provided by the United States to strike deep into Russian territory. Multiple news reports say Ukraine plans to make use of this policy reversal and conduct its first long-range attacks sooner rather than later.

"While issues of defence security don't necessarily figure prominently into the agendas ... they can't help but be connected, one way or another," retired Canadian lieutenant-general Guy Thibault, chair of the Conference of Defence Associations Institute and former vice-chief of the defence staff, said in an interview with CBC News.

"Given what we're seeing and the the concerns in terms of international insecurity that's caused by conflict, [whether] it be the Middle East, whether Ukraine, whether it be ... [what] we're seeing in the Indo-Pacific, inevitably these will be discussed," he said.

There is no guarantee Trump will not reverse Biden's decision in two months when he takes office, considering the president-elect's lukewarm support for Ukraine, which was invaded by Russia in February 2022.

"Let's ... be very blunt. All the allies in the world would not be able to replace a complete withdrawal from supporting Ukraine by the United States," Trudeau said.

That is the same challenge for any agreements made in Rio over the next two days. No one knows what will happen to those decisions on Jan. 20, when Trump is sworn in.

But even reaching consensus without Trump at the leaders' table will be a challenge. The G20 — the largest 20 economies in the world — is a diverse group of friends, rivals and even enemies.

'Escalation of tensions'

Canada's relationship with China is strained — and not likely to improve any time soon with Trump coming to power in the U.S.

"The trade wars with China will be back," said Vina Nadjibulla, vice-president of research and strategy at the Asia Pacific Foundation of Canada.

"There will be escalation of tensions on issues around Taiwan ... there will be a lot more volatility and unpredictability in the relationship. And Canada will need to watch that very closely because again, we can find ourselves caught in the middle."

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Canada's relationship with India is downright frosty. And then there's how the Canadian government feels toward Russia.

"The level of trust that I have for Vladimir Putin is probably at an all-time low right now," Trudeau said.

The Russian president will again not attend the G20 this year — relieving the host country of the awkward prospect of acting on an international warrant for Putin's arrest. In his stead, Foreign Affairs Minister Sergey Lavrov will represent Russia.

Trudeau has a number of one-on-one meetings scheduled with other leaders, including Mexican President Claudia Sheinbaum. The impending renegotiation of the free-trade agreement between Canada, the U.S. and Mexico will likely be top of mind, particularly after Trudeau did not rule out cutting Mexico out of talks if it doesn't align its tariffs on China with those of Canada and the U.S.

Trudeau will also sit down with Biden on Monday, a meeting that could be reminiscent of the farewell-like chat the prime minister had with Barack Obama at the G20 in 2016, before Trump became president the first time.