Donald Trump's most catastrophic decision

It's hard to single out any one event as the most cataclysmic of Donald Trump's presidency, in terms of American and global security. There was his bolting from the Iran nuclear deal (and Iran's subsequent resumption of uranium enrichment). There was baiting Kim Jong Un, followed by engagement, which will likely result in a more advanced North Korean nuclear program. Then, of course, there's the end to American participation in climate-change mitigation, not to mention a trade war with China.