“I know that battle damage is of great interest. Final battle damage will take some time, but initial battle damage assessments indicate that all three sites sustained extremely severe damage and destruction,” said Caine.

Later, Hegseth added that “The battle damage assessment is ongoing, but our initial assessment, as the chairman said, is that all of our precision munitions struck where we wanted them to strike and had the desired effect, which means especially the primary target here, we believe we achieved destruction of capabilities there.”

Joint Chiefs Chairman Gen. Dan Caine told reporters the mission was a “complex and high-risk mission” that was known to only a few people and relied on decoys to protect the integrity of the mission.

The B2 bombers deployed shortly after midnight on what would become the longest flight involving the B2 fleet since 2001, Caine said. A total of 14 bunker buster bombs known as MOPS were dropped, with the first two dropped at 2 a.m.

The attack also involved a U.S. submarine that launched more than two dozen Tomahawk land attack cruise missiles surface targets, according to Caine.

“This was a complex and high risk mission carried out with exceptional skill and discipline by our joint force," he said.

Iran did not deploy fighters or surface to air missiles during the mission, Caine said: “Throughout the mission, we maintained the element of surprise."

It was also the first use of the massive ordnance penetrator, or MOP, which had only been used in a testing role, according to Caine.

“This is a plan that took months and weeks of positioning and preparation so that we could be ready when the President of the United States called,” Defense Secretary Pete Hegseth said during a briefing from the Pentagon.

“It took a great deal of precision. It involved misdirection and the highest of operational security. B-2s went in and out of downtown Tehran -- not Tehran, excuse me -- up these nuclear sites in and out and back without the world knowing at all,“ said Hegseth.

“In that way, it was historic, the strike that included the longest B-2 Spirit bomber mission since 2001. And the first operational employment of the MOP a massive ordinance penetrator mission demonstrated to the world the level of joint and allied integration that speak to the strength of our alliance and our joint forces," said Hegseth.

"As President Trump has stated, the United States does not seek war. But let me be clear, we will act swiftly and decisively when our people, our partners, or our interests are threatened," Hegseth also said.

Defense Secretary Pete Hegseth is briefing reporters on the U.S. strike on Iran, dubbed Operation Midnight Hammer, declaring that “we’ve devastated the Iranian nuclear program” and that Iran’s nuclear program has been “obliterated.”

Hegseth called the military strike “bold and brilliant.”

“When this president speaks, the world should listen,” he said.

Amid ongoing aerial attacks against Israel, Iran's Supreme Leader Ali Khamenei said the country must show "no mercy."

"We must give a strong response to the terrorist Zionist regime," Khamenei said, adding, "We will show the Zionists no mercy."

-ABC News' Will Gretsky