About USA breaking the Iran Nuclear Deal, read this from a 2009 Brookings Institution paper:
"…any military operation against Iran will likely be very unpopular around the world and require the proper international context—both to ensure the logistical support the operation would require and to minimize the blowback from it.
The best way to minimize international opprobrium and maximize support (however, grudging or covert) is to strike only when there is a widespread conviction that the Iranians were given but then rejected a superb offer—one so good that only a regime determined to acquire nuclear weapons and acquire them for the wrong reasons would turn it down. Under those circumstances, the United States (or Israel) could portray its operations as taken in sorrow, not anger, and at least some in the international community would conclude that the Iranians “brought it on themselves” by refusing a very good deal."

Source: "Which Path to Persia?…" PDF: http://qttr.at/23nj