When the United States began deploying a missile defense system in South Korea this week, it was to protect an ally long threatened by North Korean provocations. But it was instantly met by angry Chinese warnings that the United States is setting off a new arms race in a region already on edge over the North’s drive to build a nuclear arsenal.

China condemned the new antimissile system as a dangerous opening move in what it called America’s grand strategy to set up similar defenses across Asia, threatening to tilt the balance of power there against Beijing.

The tensions are testing the new Trump administration and its uneasy allies South Korea and Japan, which have complained for years that China has simultaneously chastised and coddled the North, refusing to enact stiff enough measures to force it to abandon its nuclear and missile programs.

But with the beginning of work to install the antimissile system, the delicate international cooperation against North Korea is splintering: Beijing is expressing more concern about American intentions in the region than about the dangers of the North’s latest surge in nuclear and missile testing.