A resolution to encourage breastfeeding was expected to be approved quickly and easily by the hundreds of government delegates who gathered this spring in Geneva for the United Nations-affiliated World Health Assembly. Based on decades of research, the resolution says that mother’s milk is healthiest for children and countries should strive to limit the inaccurate or misleading marketing of breast milk substitutes.

Then the U.S. delegation, embracing the interests of infant formula manufacturers, upended the deliberations.

American officials sought to water down the resolution by removing language that called on governments to “protect, promote and support breastfeeding” and another passage that called on policymakers to restrict the promotion of food products that many experts say can have deleterious effects on young children.

When that failed, they turned to threats, according to diplomats and government officials who took part in the discussions. Ecuador, which had planned to introduce the measure, was the first to find itself in the cross hairs.

The Americans were blunt: If Ecuador refused to drop the resolution, Washington would unleash punishing trade measures and withdraw crucial military aid. The Ecuadorean government quickly acquiesced.

The showdown over the issue was recounted by more than a dozen participants from several countries, many of whom requested anonymity because they feared retaliation from the United States.

Health advocates scrambled to find another sponsor for the resolution, but at least a dozen countries, most of them poor nations in Africa and Latin America, backed off, citing fears of retaliation, according to officials from Uruguay, Mexico and the United States.

“We were astonished, appalled and also saddened,” said Patti Rundall, policy director of the British advocacy group Baby Milk Action, who has attended meetings of the assembly, the decision-making body of the World Health Organization, since the late 1980s.

“What happened was tantamount to blackmail, with the U.S. holding the world hostage and trying to overturn nearly 40 years of consensus on the best way to protect infant and young child health,” she said.

In the end, the Americans’ efforts were mostly unsuccessful. It was the Russians who ultimately stepped in to introduce the measure — and the Americans did not threaten them.

The State Department declined to respond to questions.

The intensity of the administration’s opposition to the breastfeeding resolution stunned public health officials and foreign diplomats, who described it as a marked contrast to the Obama administration, which largely supported WHO’s long-standing policy of encouraging breastfeeding.

During the deliberations, some American delegates even suggested the United States might cut its contribution to the WHO, several negotiators said. Washington is the single largest contributor to the health organization, providing $845 million, or roughly 15 percent of its budget, last year.

A Russian delegate said the decision to introduce the breastfeeding resolution was a matter of principle.

“We’re not trying to be a hero here, but we feel that it is wrong when a big country tries to push around some very small countries, especially on an issue that is really important for the rest of the world,” said the delegate.