WASHINGTON — After decades of maintaining a minimal nuclear force, China has re-engineered many of its long-range ballistic missiles to carry multiple warheads, a step that federal officials and policy analysts say appears designed to give pause to the United States as it prepares to deploy more robust missile defenses in the Pacific.

What makes China’s decision particularly notable is that the technology of miniaturizing warheads and putting three or more atop a single missile has been in Chinese hands for decades. But a succession of Chinese leaders deliberately let it sit unused; they were not interested in getting into the kind of arms race that characterized the Cold War nuclear competition between the United States and the Soviet Union.

Now, however, President Xi Jinping appears to have altered course, at the same moment that he is building military airfields on disputed islands in the South China Sea, declaring exclusive Chinese “air defense identification zones,” sending Chinese submarines through the Persian Gulf for the first time and creating a powerful new arsenal of cyberweapons.

Many of those steps have taken American officials by surprise and have become evidence of the challenge the Obama administration faces in dealing with China, in particular after American intelligence agencies had predicted that Mr. Xi would focus on economic development and follow the path of his predecessor, who advocated the country’s “peaceful rise.”