As reports emerged that the South Korean government was surprised by the announcement, questions were raised about whether Mattis and other U.S. policymakers had also been caught off guard.
“There were no surprises, they had spoken on all of these issues in advance,” Dana White, the chief Pentagon spokesperson, told ABC News when asked if Mattis knew about the president's announcement.
In an exclusive interview with ABC News's George Stephanopoulos and at a press briefing Trump announced that he was suspending military exercises conducted annually in South Korea by the U.S. military.
"I’m doing something that I’ve wanted to do from the beginning," Trump said in the interview with Stephanopoulos. "We stopped playing those war games that cost us a fortune."
"We’re spending a fortune, every couple of months we’re doing war games with South Korea," he added.
Asked if Mattis was surprised by those comments White said Mattis had been in constant conversations with the President.
“He was not surprised, he was consulted,“ she added. "They’d been having lots of conversations, but about all things and possibilities. “
“These are negotiations so the Secretary has been in full consultation with Secretary of State Pompeo, as well as the president,” said White.
“The Secretary’s in full alignment with the President to meet his goal which is denuclearization of the Peninsula.”
In a formal statement, White said the Pentagon welcomed "the positive news coming out of the summit and fully supports the ongoing, diplomatically-led efforts with the Democratic People's Republic of Korea."
"Our alliances remain ironclad, and ensure peace and stability in the region," she said in the statement. "The Presidential summit outcome is the first step along the path to the goal: complete, verifiable and irreversible denuclearization of the Korean Peninsula and a free and open Indo-Pacific."