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.