Speaking on ABC's "This Week with George Stephanopoulos," the United States' top diplomat said that is U.S. policy "to the extent that we have alliances and understandings with a number of nations."
"They may not be formal, as it is with NATO, but I don't think there is any doubt in anyone's mind that, were Israel to suffer a nuclear attack by Iran, there would be retaliation," Clinton said.
When asked specifically if that counterattack would be by the United States, she answered, "Well, I think there would be retaliation." Israel is known to have scores of nuclear weapons of its own.
Clinton went on to say the United States wants to avoid a Middle East arms race that "leads to nuclear weapons being in the possession of other countries in the Middle East, and we want to make clear that there are consequences and costs."
She also said the United States has to "make clear to the Iranians is that their pursuit of nuclear weapons will actually trigger greater insecurity" in the region.
In regard to U.S-Iran relations, Clinton said she thinks there is "an enormous amount of potential for change, if the Iranians are willing to pursue that." But, she said, the United States needs better information from Iran if there is to be meaningful change.
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