At the talks tomorrow and Friday in the western city of Bonn, US allies are seeking reassurances from Rex Tillerson that the administration of US President Donald Trump won't ditch a decade of close cooperation among G-20 nations on climate change, international development and the global economy.
Russia, China, Saudi Arabia and Turkey, meanwhile, are looking to gauge the man they'll be dealing with on some of the more sensitive areas of foreign policy in the coming years -- including conflicts in Syria and Ukraine, territorial disputes in the South China Sea and nuclear disarmament.
A shift away from multilateral diplomacy could see US allies pitted against each other in a bid for Washington's attention, opening up new battle lines. At the same time, smaller countries could be left to pick up the cost of financing international organizations -- as well as the burden of behind-the-scenes negotiations -- previously shouldered largely by the State Department.
More From This Section
How that will work in practice, and who genuinely has the new president's ear, is something diplomats will be trying to find out both in the summit's two working sessions and during a flurry of bilateral meetings at a German government guest house on a mountain overlooking the River Rhine.
"Russia would be a very interesting conversation to hear," said Smart, noting that the G-20 meeting comes days after Trump's national security adviser Michael Flynn resigned over his contacts with Moscow.
German Chancellor Angela Merkel is trying to downplay differences with Trump -- who called her refugee policy "a total disaster" -- by stressing Berlin's willingness to cooperate with his administration.
Earlier this month, she sent her foreign minister, Sigmar Gabriel, to Washington to meet with Tillerson within hours of his Senate confirmation.