SEOUL, South Korea — President Xi Jinping of China arrived in South Korea on Thursday for a state visit to a vital American ally, a move that appears to signal his resolve to unsettle America’s alliances in Northeast Asia and fortify his argument for a new security architecture in the region, with China as the dominant player.

In the past, Chinese leaders have visited their ally North Korea before visiting South Korea, but Mr. Xi seems to be showing the North Korean leader, Kim Jong-un, the cold shoulder: He has not been to Pyongyang, nor has he invited Mr. Kim to Beijing.

Instead, he is embracing President Park Geun-hye of South Korea, where $270 billion in trade with China and Ms. Park’s frosty relations with Japan, America’s prime ally in Asia, offer more prospects for Beijing’s ambitions to undermine the United States.

Mr. Xi is visiting Seoul in the wake of Japan’s decision on Tuesday to reinterpret its pacifist Constitution to allow its military to play a more assertive role in the region. The announcement is deeply unpopular here and in China, its intended target.