But with a mix of secrecy and vague assurance, Mr. Sogavare has shaken his own democracy and the stability of the entire Asia-Pacific region. Having already suggested that he wants to delay next year’s election by rewriting the constitution, the prime minister now has China to lean on if protests break out. At the same time, China’s leader, Xi Jinping, and his army now have a foothold in an island chain that played a decisive role in World War II and could be used to block vital shipping lanes.

“It’s a game changer,” said Anne-Marie Brady, a professor at the University of Canterbury in Christchurch, New Zealand, who has investigated Chinese influence in the region.

The deal — if the signed version, as expected, resembles the draft — reveals a stunning set of potential precedents for world leaders who are already losing sleep over the global contest between democracy and autocracy.