South Korea’s security strategy has rested for more than seven decades on a single pillar: its alliance with the United States. That pillar has absorbed one blow after another during Donald Trump’s second term as US president. This week brought a fresh signal that has strained the relationship. In a social media post, Trump criticised joint US-South Korea military exercises, calling them “costly” and saying they send an “inappropriate and hostile” message to a North Korea he described as...