STUNG TRANG, Cambodia — As the sun rose over the murk of the Mekong River, the man who has ruled Cambodia for more than three decades, Prime Minister Hun Sen, clasped hands with the Chinese ambassador and beamed.

“The Chinese leaders respect me highly and treat me as an equal,” Mr. Hun Sen said last month during the groundbreaking of a $57 million Chinese-funded bridge in the district of Stung Trang.

“Let me ask those of you who have accused me of being too close to China,” he added. “What have you offered me besides cursing and disciplining me and threatening to put sanctions on me?”

For a quarter-century, the West helped rebuild Cambodia while it was still reeling from the genocidal Khmer Rouge. The United States and Europe tied billions of dollars in aid to an effort to transform Cambodia into a liberal democracy.