"I would say: If you go into Taiwan, I'm sorry to do this, I'm going to tax you, at 150 per cent to 200 per cent," the former US president was quoted as saying in an interview with the newspaper.
Trump, asked if he would use military force against a blockade on Taiwan by China, said it would not come to that because Chinese President Xi Jinping respected him.
"I had a very strong relationship with him," Trump said. "I wouldn't have to (use military force), because he respects me and he knows I'm f***ing crazy," he said in the interview.
China claims democratically governed Taiwan as its own territory and has never renounced the use of force to bring the island under its control. Taiwan strongly objects to China's sovereignty claims.
Trump, as part of his pitch to voters in the knife-edge November 5 election in which he faces Vice President Kamala Harris, has floated plans for blanket tariffs of 10 per cent to 20 per cent on virtually all imports as well as tariffs of 60 per cent or more on goods from China, in measures he says would boost US manufacturing.
During his term as president from early 2017 to early 2021, Trump's aggressive approach toward China was underscored by waves of tariffs that plunged the two countries into a trade war that moved markets worldwide.
In the interview with the Wall Street Journal, Trump also spoke about Russia's invasion of Ukraine, repeating his claim that if he were still in office Russian President Vladimir Putin would not have launched the invasion.
"I said to Putin, 'Vladimir, we have a great relationship. ... Vladimir, if you go after Ukraine, I am going to hit you so hard, you're not even going to believe it. I'm going to hit you right in the middle of fricking Moscow'," Trump was quoted as saying when talking about a past interaction with Putin.
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