"This deal is not built on trust. It is built on verification," the president said from the White House early Tuesday, returning to a refrain he and his negotiators have used repeatedly during the talks.
After nearly two years of discussions, negotiators from Iran and six world powers, including the U.S., finally agreed to a deal designed to prevent Iran from developing a nuclear weapon for at least a decade in exchange for giving Tehran relief from crippling economic sanctions.
"This deal demonstrates that American diplomacy can bring out real and meaningful change," Obama said.
The restrictions are designed to extend the time it would take to produce a nuclear bomb if Iranian leaders decided to try to build one. The "breakout period," now estimated at two to three months, would stretch to at least a year, according to U.S. analysis of the plan. Obama vowed that "every pathway to a nuclear weapon is cut off."
"Because America negotiated from a position of strength and principle, we have stopped the spread of nuclear weapons in this region," he said. "Because of this deal, the international community will be able to verify that the Islamic Republic of Iran will not develop a nuclear weapon."
Announcement of the agreement is all but certain to set off an intense debate within the U.S. and internationally. As Obama prepared to deliver his early-morning remarks, critics in Congress were already warning of potential pitfalls.
Sen. Ben Sasse (R-Neb.) said the agreement "lit the fuse" for a nuclear arms race in the Middle East.
"This deal abandons America's historic bipartisan commitment to preventing nuclear proliferation, and instead begins the era of managed proliferation -- a descent into chaos and an even more dangerous world," he wrote in an email.
But Obama sees the deal as a signal achievement that could avert tragic outcomes. He has never taken military action off the table as an option for preventing Iran from building a nuclear weapon.
If Iran were to continue its drive toward a nuclear weapon, he said, other nations in the region would begin to do so as well.
"No deal means a greater chance of more war in the Middle East," Obama said, pledging to veto any attempt by Congress to block implementation of the agreement.
In making his case, Obama cited Cold War-era negotiations with the Soviet Union, which, he noted, was committed to the destruction of the United States at the time. Those negotiations ended up making the U.S. safer, he said.