The election of a populist, nativist, nationalist, isolationist Donald J. Trump to the presidency of the US shook the global architecture in 2016. It signalled the end of Pax Americana, the end of American leadership of the global rules-based order and the beginning of a new phase in geopolitical diplomacy and G20 summitry, one characterised by disputes and disagreements, and splits with allies. The US administration’s efforts to implement America First have different effects. At the one extreme it means the evisceration and crippling of the World Trade Organization dispute settlement system. G20 summits became exercises in diminished expectation and dispute management, with America standing opposition to collective goals. In other policy areas the impact is lessened by countervailing influences, complexities, institutions and dynamics. The impact of America First can be visualised on a matrix which allows the reader to judge a policy’s likely impact depending on key variables. The long-term effect of America First cannot yet be fully determined. But a second term for the American pugilist Mr Trump would certainly be highly detrimental to the rules-based international order.