“For as much as Trump is mocked and ridiculed, he has played a forcing function in this discussion,” Mr. Bannon said. “Many of the elites, and many of the institutions, have become more much hawkish about China. There are very few cheerleaders anymore.”

“It is Trump and his collection of nationalists who have brought this to the forefront,” he added.

Mr. Bannon was the standard-bearer for the nationalists during the 2016 campaign and as the president’s chief strategist before he was ejected from the White House last summer. Though he initially stayed in Mr. Trump’s good graces — talking to him regularly — the president turned sharply against him after the publication of “Fire and Fury,” Michael Wolff’s account of the White House, in which Mr. Bannon is quoted disparaging the president, his daughter Ivanka and his son-in-law, Jared Kushner.

“When he was fired,” the president said in January, “he not only lost his job, he lost his mind.”

In exile, Mr. Bannon has sought to restyle himself as a kingmaker in Republican politics, backing nationalist candidates in congressional races around the country. But he was dealt a stinging setback in Alabama, after his scandal-scarred protégé, Roy S. Moore, was defeated in a special election for the Senate by the Democrat, Doug Jones.

While Mr. Bannon and Mr. Trump have yet to repair their rift, the president’s moves against China represent a policy victory for his excommunicated aide. During his final weeks in the White House, Mr. Bannon kept a whiteboard in his office, on which he had scrawled a calendar for trade actions against China, the European Union and others.

The rollout was delayed for a few months after other advisers, including Treasury Secretary Steven Mnuchin and Gary D. Cohn, the director of the national economic council, persuaded Mr. Trump that he should not impose tariffs at the same time he was pushing a tax-cut bill, because it would antagonize Republicans on Capitol Hill.

Later, Mr. Cohn and Mr. Mnuchin warned the president that protectionist moves would pummel the stock market, which Mr. Trump views as a barometer of his success. That was true last week, when the market plunged over fears of a trade war with China. But Mr. Trump’s more conciliatory tone over the weekend fueled a modest rebound on Monday.

Mr. Bannon blamed the news media and Wall Street for fanning fears of a trade war, even as he conceded that the tariffs would drive up the price of some consumer goods in the United States and that China’s retaliation against soybean imports could hurt American farmers.