WASHINGTON — Two weeks ago, Donald J. Trump said he could live with a nuclear-armed Japan and South Korea if it meant they could defend themselves against North Korea without American aid. “I’m not sure that would be a bad thing for us,” he said.

Since then he has changed his tune. After Japanese and South Korean officials raised fears of an Asian arms race, and President Obama ridiculed his remark, Mr. Trump began to say he did not actually want the two countries to obtain nuclear weapons — but that, because of American weakness, “at some point it could happen anyway.”

It was not the first time Mr. Trump has hastily added deflating caveats to his headline-grabbing trial balloons.

In a debate a month ago, he declared himself in favor of torture if it would extract information from terrorists, then issued a statement saying he would respect the law, then followed it up by saying that the law must be changed.