U.S. Secretary of State John F. Kerry speaks at a Reuters Newsmaker event Tuesday in New York on the nuclear agreement with Iran. (Brendan McDermid/Reuters)

Three dozen retired generals and admirals released an open letter Tuesday supporting the Iran nuclear deal and urging Congress to do the same.

Calling the agreement “the most effective means currently available to prevent Iran from obtaining nuclear weapons,” the letter said that gaining international support for military action against Iran, should that ever become necessary, “would only be possible if we have first given the diplomatic path a chance.”

The release came as Secretary of State John F. Kerry said U.S. allies were “going to look at us and laugh” if the United States were to abandon the deal and then ask them to back a more aggressive posture against Iran.

Not only would U.S. global credibility be undermined, Kerry said, but also the dollar’s position as the world’s reserve currency would be threatened.

“It’s not going to happen overnight,” Kerry said in a public question-and-answer session at Reuters news service headquarters in New York . “But I’m telling you, there’s a huge antipathy out there” to U.S. leadership. Pointing to efforts by Russia and China to join forces with rising, nonaligned powers, he said that “there’s a big bloc out there, folks, that isn’t just sitting around waiting for the United States to tell them what to do.”

Kerry and President Obama, who is vacationing in Martha’s Vineyard, are using the August congressional recess to counter claims made by opponents of the deal during recent hearings.

People who think negotiators can go back to the drawing board and improve on what has been agreed are unrealistic, Kerry said.

“When I hear a senator, a congressman stand up and say ‘We should get a better deal’ — That is not going to happen,” he said. “If everybody thinks ‘Oh, no, we’re just tough. . . . we can force people. . . . America is strong enough, our banks are tough enough, we can just bring the hammer down and force people to do what we want to do.’

“Are you kidding me?” Kerry said.

Instead, he painted a harsh picture of the results of U.S. rejection. Allies would refuse to retain sanctions or impose new ones, or join in possible military action, he said.

The letter from the retired military officers followed the release this past weekend of a letter to Obama by 29 of the nation’s leading scientists, who called the Iran deal “technically sound, stringent and innovative” and said it would “provide the necessary assurance in the coming decade and more that Iran is not developing nuclear weapons.”

The letters provide the White House with additional backing as it wages an increasingly uphill fight to protect the agreement from congressional destruction. Lawmakers will decide next month whether to “disapprove” the deal, a vote that currently appears sure to win near universal Republican support and a significant number of Democratic defections.

The administration’s fight now is to persuade enough Democrats to vote to sustain an Obama veto of the disapproval. Some Democratic lawmakers have already said they favor the deal while others, including Sen. Charles E. Schumer (D-N.Y.), in line to be the next Democratic leader in the Senate, have voiced opposition.

Under a deal negotiated between the White House and Congress, if a disapproval resolution stands, Obama will be barred from waiving U.S. sanctions as part of U.S. responsibility under the agreement.

Signers of the military letter include retired general and flag officers from every branch of service. They include four-star Marine Gens. James Cartwright, former vice chairman of the Joint Chiefs of Staff, and Joseph P. Hoar, former head of the U.S. Central Command; and Gens. Merrill McPeak and Lloyd W. Newton of the Air Force.

“There is no better option to prevent an Iranian nuclear weapon,” the letter said. “Military action would be less effective than the deal, assuming it is fully implemented. If the Iranians cheat, our advanced technology, intelligence and the inspections will reveal it, and U.S. military options remain on the table.”

“And if the deal is rejected by America,” it said, “the Iranians could have a nuclear weapon within a year. The choice is that stark.”

Retired Navy Rear Adm. Harold L. Robinson, a rabbi and former naval chaplain who chairs the National Conference on Ministry to the Armed Forces, also signed.

“As a lifelong Zionist, devoted to Israel, and a retired general officer and a rabbi for over 40 years, and operating without institutional encumbrances, I have a unique perspective,” Robinson said in an interview.

He said he spoke out to demonstrate that “those of us who love Israel in the United States are not of one mind and one voice on this matter. I thought it was important to represent some of the diversity within the American Jewish community.”

The Israeli government is adamantly opposed to the agreement, and the American Israel Public Affairs Committee has been in the forefront of a campaign to build public opposition in this country.