BEIJING — Having cast aside presidential term limits, China is bracing for relations with the United States to enter a dangerous period under the continuing leadership of President Xi Jinping, intending to stand firm against President Trump and against policies it sees as attempts to contain its rise, according to Chinese analysts.

Even before the announcement on Sunday that he could rule for the foreseeable future, Mr. Xi had ordered the Chinese military to counter the Pentagon with its own modernization in air, sea, space and cyber weapons, the analysts said, partly in response to Mr. Trump’s plans to revitalize American nuclear forces.

Rather than beginning a final term next month as a lame duck, Mr. Xi will govern with new authority to pursue his agenda of making China a global power even if it risks putting Beijing in conflict with Washington and triggering a new Cold War after 40 years of mutual engagement, the analysts said.

“In the Asia-Pacific, the dominant role of the United States in a political and military sense will have to be readjusted,” said Cui Liru, former president of the China Institutes of Contemporary International Relations, a think tank under the Ministry of State Security that often reflects official thinking. “It doesn’t mean U.S. interests must be sacrificed. But if the U.S. insists on a dominant role forever, that’s a problem.”