The United States, in fact, sought to stop Italy’s joining of the Silk Road.

This week Secretary of State Mike Pompeo said he was “saddened” by the development. On Thursday, Luigi Di Maio, Italy’s deputy prime minister and the political leader of the Five Star Movement, met with National Security Adviser John R. Bolton at the White House.

Mr. Di Maio told reporters that he assured the Americans that the China deal was purely about commerce and that Italy remained firmly in the political orbit of the United States, which has vastly more money invested in the country than China does.

In the months before the deal, Mr. Di Maio repeatedly visited China, and nearly made a deal in November, Italian officials said. All along, the Italians said, they heard barely a peep from top officials in the United States, and by the time Mr. Bolton’s spokesman went public against the deal earlier this month, it was too late.

A senior government official, speaking on background to discuss internal deliberations, said Washington would have engaged sooner if it had understood that Italy planned to officially join the new Silk Road, which it sees as a strategic threat.

The official said the chaotic nature of Italian politics and the fact that visits to Beijing had become the norm for European leaders made it harder to discern what the Italians were up to.

Indeed, already 16 central and eastern European countries, including 11 members of the European Union, have formal business relations with China.

The Chinese have essentially bought the port of Piraeus, outside Athens. The new deal will now also allow it access to critical Italian ports, like Genoa and another in Trieste, which has a rail link reaching right into the heart of Central Europe.