Jeffrey Sachs is a professor and director of the Center for Sustainable Development at Columbia University. The opinions expressed in this commentary are his own.

President Donald Trump's bizarre instability was on full display at the G7 Summit in France this weekend. When asked whether he had second thoughts about the recent escalation of the trade war with China, he first answered affirmatively, leading to headlines around the world that Trump had softened his position on China. Hours later, his aide clarified that Trump regretted that he hadn't raised tariffs on Chinese goods even higher. Then at his closing G7 press conference, Trump again struck a conciliatory note, claiming that the Chinese had called US trade officials to return to negotiations, while the Chinese foreign ministry spokesperson said that they were unaware of any such call.

Trump is a one-man wrecking ball deliberately smashing the rules of global trade, finance, environment and diplomacy. Trump swings back and forth between fury and conciliation, one day calling Chinese President Xi Jinping an enemy and days later a "great leader." If past is prologue, Trump's conciliation today will soon give way to new vitriolic attacks, especially since he naively believes China is about to cave to his high-pressure tactics. China has repeatedly stated it will not cave.

Congress must urgently reassert its constitutional powers before Trump's erratic behavior ruins the US and world economy and further endangers the planet both environmentally and diplomatically.

Trump has isolated the United States on a rapidly expanding list of major international issues. He has threatened and continues to threaten unilateral tariffs on US allies on the absurd grounds that they are threatening US national security. He unilaterally broke the Iran nuclear agreement to the deep dismay of America's European allies and invoked extraterritorial US sanctions on Iran strongly opposed by the most of the world. He announced the pending US withdrawal from the Paris Agreement on climate change, the only nation of the 195 signatories to do so. He has injured global trade and business investments by attempting recklessly and provocatively to undermine China's economy.

Last Friday, Trump tweeted that "Our great American companies are hereby ordered to immediately start looking for an alternative to China, including bringing your companies HOME and making your products in the USA." The Dow immediately fell by more than 600 points. There is more financial instability to come.

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