FULLY SEVEN weeks before he is due to take office, President-elect Donald Trump launched what looked like an offensive against China beginning last week. First came a precedent-breaking phone call with the president of Taiwan; then came a series of tweets assailing China’s trade and currency policies and its buildup in the South China Sea. Mr. Trump’s rhetoric was not new, and his apparent strategy of pushback against the regime of Xi Jinping has some merit. What’s worrying is the evident lack of preparation and diplomatic care in the initiative, as well as the unintended consequences it may produce.

On Friday, in a move that was reportedly planned, Mr. Trump took a congratulatory phone call from Taiwanese President Tsai Ing-wen, breaking with decades of U.S. policy. When the United States formally opened diplomatic relations with China in 1979, Taiwan was relegated to nondiplomatic status, which has meant arms sales and support but not phone calls or meetings at the highest level. China views Taiwan as a breakaway province and has always been hypersensitive to any quiver in its standing in the world, and especially its ties to the United States.

The phone call predictably raised alarms in Beijing. By itself, a courtesy call does not seem so earthshaking to us, given that Taiwan is a thriving democracy with a vibrant civil society and is an important U.S. trading partner. Yet it may produce countermoves from Beijing — such as new economic and military pressures on Taiwan — that may undercut the call’s political boost to Taipei while further stoking already-high tensions in East Asia.

The president-elect did not stop with the phone call. Next came Twitter messages on Sunday that echoed his campaign blasts against China on economic issues and the South China Sea. The reality of these issues is far more complex than Mr. Trump’s tweets allow. But more importantly, they carried a tone of aggressive challenge. Do they mean Mr. Trump will plunge the United States into a trade war with China? He must realize that such a confrontation could prove counterproductive and a serious drag on his hopes to boost economic growth at home. Is he planning steps against China’s “massive military complex in the middle of the South China Sea”? If not, his rhetoric may look hollow in a few months.

Mr. Trump takes pride in being undiplomatic and unpredictable. But if he wants to effectively challenge China, a rash of Twitter messages hardly seems the right way to go about it. He has been acting without the benefit of U.S. intelligence briefings or advice from the State Department, and his weekend missives were apparently uncoordinated with the current administration. His impulsive statements carry the risk of misunderstanding and miscalculation.

Aggravating China also could have a downside when Mr. Trump needs to ask Beijing for help with its errant client state, North Korea. Kim Jong Un’s accelerating nuclear weapons and missile programs will be near the top of Mr. Trump’s problems upon taking office. China is an essential player in restraining North Korea. This is just one example of the costs and benefits that Mr. Trump should weigh — preferably with experienced advisers — before letting fly on Twitter.