BEIJING >> After unleashing an anti-U.S. tirade in Beijing last week, President Rodrigo Duterte of the Philippines left some Chinese, who like tightly scripted state visits, wondering if their unpredictable guest could be trusted.

Yet his alacrity in accepting direct talks over the disputed South China Sea, and his gratitude for $24 billion in investment and financing deals, also left the broad impression that China may have started a strategic realignment in Southeast Asia by bringing an important U.S. ally to its side.

Countries like Vietnam, which had been edging closer to the United States, and Malaysia and Thailand, which were moving toward Beijing, may now see the value in drawing closer to China, analysts said.

“China has improved relations with Duterte immediately, and set up a way to settle the South China Sea disputes peacefully,” said Yan Xuetong, a professor of international relations at Tsinghua University and a prominent foreign-policy hawk. “Generally speaking, this problem in the South China Sea is over, and the United States cannot do anything now.”

The Philippines, a treaty ally of the United States since 1951, has been regarded as one of the fundamental pro-American countries in Asia.

In his declaration that he was seeking a “separation” from the United States — a statement that he scaled back when he got home — Duterte seemed to signal that he was dumping his country’s longtime ally and former colonial power.

He also said he would form a triple alliance with China and Russia to “face the world.”

Some analysts marveled at Beijing’s good fortune, after a tense period leading to a ruling by an international tribunal in The Hague against China about its activities in the South China Sea. And Duterte’s apparent about-face came without the leadership in Beijing doing much more than accepting the results of the democratic election that put him in office.

“It’s a gift, not a victory,” Yan said of Duterte’s remarks. “Victory means you get something through your own effort. We did nothing. It’s a gift.”

At the same time, Duterte’s hosts were startled by his anti-U.S. remarks, raising concerns that he could become a liability.

There is little chance, for example, that Beijing policymakers would want a China-Russia-Philippines alliance, said Zhang Baohui, a professor of international relations at Lingnan University in Hong Kong.

“I don’t think at this moment Beijing wants a really close partnership with him, especially if that partnership could be seen as an anti-U.S. alliance,” Zhang said.

On social media, there was skepticism about Duterte’s sincerity. Perhaps he was engaging in a game like one played by couples in Shanghai, who arrange fake divorces to evade restrictions on property purchases, one person wrote on Weibo.

Others on social media made fun of Duterte as the ruler of a country that is often belittled here as a place for abundant tropical fruit but not much more. One cartoonist depicted Duterte as a banana salesman who had plenty for sale now that the embargo of the past four years had been lifted.

Still, the Philippine delegation’s visit was viewed as a success on several fronts.

The Philippine defense secretary, Delfin Lorenzana, met with his Chinese counterpart, Chang Wanquan, and agreed to a renewal of ties between the two navies. There were no specifics on what the new relationship would entail, but the encounter struck a different tone to the strained relationship under President Benigno S. Aquino III.

Another gain for China: the acceptance by the Philippines of large amounts of infrastructure to be built by state-owned Chinese companies. If the deals come to fruition, they will significantly reverse China’s relatively small economic presence in the Philippines.

One of the deals that is likely to give China particular satisfaction is a pledge by the state-owned CCCC Dredging to enlarge the Cebu International and Bulk Terminal port.

That company carried out most of the reclamation for creating the artificial islands in the Spratly archipelago in the South China Sea that the former Philippine government protested in its recent case against China at The Hague.

The Philippine trade secretary, Ramon Lopez, said that China had agreed to spend $15 billion on projects to help achieve the biggest infrastructure boom in the Philippines since the authoritarian rule of President Ferdinand Marcos.

Trinh Nguyen, an economist specializing in the Asian Pacific region at Natixis Bank, sounded a note of caution about the value of the Chinese investments.

“Pledges do not necessarily translate into realized foreign direct investment for the Philippines,” he said. “This is just a symbol of the Chinese government’s good will toward the country.” In Indonesia, for example, China outbid Japan on a high-speed train line, but the project has been badly delayed and yet to deliver results, he said.

Despite Duterte’s anti-U.S. stance in Beijing, he refrained from specifically stating that he would halt U.S. access to five Philippine bases. In the longer term, China would relish such a policy because it would further the country’s goal of control of the South China Sea and complicate U.S. military planning.

But such a pledge was not expected on the first visit, and China was satisfied with Duterte’s decision to accept the plan for direct talks on the South China Sea, a position that is contrary to U.S. policy, analysts said.

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The United States has argued that disputes over territory and fishing rights in the waterway should be settled among all the countries involved in the dispute, and preferably under the auspices of the regional group, the Association of Southeast Asian Nations.

Zhang, the Lingnan University professor, said Duterte had “confirmed that the bilateral approach does work better.”

“This lends major support to China,” Zhang said.

He also noted that the U.S. strategy needed a “victim” of China to rally other countries around, but that Duterte had removed the Philippines from that posture.

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There is no doubt that Duterte’s success with China has rattled Southeast Asian nations who seek its economic largess but fear its strategic ambitions, said Alexander L. Vuving, a professor of international relations at the Daniel K. Inouye Asia-Pacific Center for Security Studies in Hawaii.

Vietnam is now being forced to rethink its stance of leaning toward the United States, he said. But more likely than tilting toward China, Vietnam will seek to persuade Washington to strengthen its commitment to the region, he said.

“Duterte’s pivot causes confusion,” Vuving said, “but also validates Vietnam’s current approach to China, which combines deference with resistance and avoids relying on a single great power.”