Morgan Ortagus, the department's spokesperson from 2019-2021, described the exclusion from the meeting between then President Donald Trump and North Korean leader Kim Jong-un as an "America first foreign policy in action," stressing that though the US works with allies, it will sometimes move "independently" when it serves America's interests, Yonhap news agency reported.
She wrote a chapter in the book titled "An America First Approach to US National Security." It was published by the America First Policy Institute (AFPI), a non-profit organisation. Her chapter was titled "America First, NATO, and US Alliances: Why America First is Not Isolationism."
"The United States also deliberately excluded Moon from the Singapore summit because he was too willing to make concessions to North Korea," she wrote.
"This is an America-first foreign policy in action: American strength, presidential leadership, peace through strength, and working with allies with the caveat that the United States will sometimes act independently when it is in our nation's interest," she added.
The Singapore summit took place in June 2018, producing an agreement under which Pyongyang committed to work toward the "complete denuclearisation" of the Korean Peninsula, while both sides agreed to work together to build new relations and foster a lasting, stable peace regime on the peninsula.
Ortagus noted that the US "listened to" Moon but took a much harder line against Pyongyang than Moon wanted.
"Although the United States worked closely with Japan and South Korea in its approach to North Korea, the Trump administration did not give either state a veto over America's North Korea policy," she said.
She portrayed Trump's personal diplomacy with the North Korean leader as a "case study" of how an America-first foreign policy can work.
In particular, she recalled Trump's UN address in 2017, in which he said that the US has great strength and patience, but if forced to defend itself and its allies, it would have no choice but to "totally destroy" North Korea.
"Trump's tough rhetoric, sanctions, and policies against North Korea in 2017 led Kim to agree to diplomacy with the United States, including a summit with the president in Singapore in June 2018," she said.
She took a swipe at President Joe Biden's foreign policy, saying his administration has continued along a "dangerous" path.
"North Korea's significant expansion of its missile program since 2021, along with possible preparations for a seventh underground nuclear test, has seen no real policy response from the Biden administration," she said.
"This has been driven by the president naming a part-time North Korean special envoy and a failure of top Biden officials to attempt bilateral diplomacy with their North Korean counterparts."
Calling Biden's Asia foreign policy "weak," she said it has led to joint naval and air exercises between Russia and China and a "new Russia-China axis that has been augmented by North Korea and Iran."
Ortagus touched on South Korean President Yoon Suk Yeol's move to cope with evolving North Korean threats while pointing to Yoon's past comment hinting at the possibility of South Korea exploring its own nuclear option.
"Yoon's comments caused panic in Biden's national security team and led to a crash program to repair US relations with South Korea and Japan, culminating with the Biden-Yoon summit and the Camp David summit," she said.
The Camp David summit refers to a landmark trilateral meeting between Yoon, Biden and Japanese Prime Minister Fumio Kishida at the presidential retreat in Maryland in August. It produced a series of agreements, including the "Commitment to Consult" each other in the event of a shared threat.
"Although Biden deserves credit for these efforts, it is fair to say the meetings were a response to his administration's neglect toward our Asian allies and their worries about whether the United States would support them against China and North Korea," she said.
In a chapter written by Stephen Yates and Adam Savit of the AFPI, they noted the role of the 28,500-strong US Forces in Korea in dealing with potential threats from China.
"With the Korean Peninsula frozen in conflict since 1953, nearly 30,000 US troops are stationed in the South alongside a formidable South Korean military that stands to deter the nuclear-armed North," they wrote.
"These forces are crucial to deterring China's likely attempts to goad Kim Jong Un's regime into mounting a Korean front in the event of a general conflict, but this would likely leave them unable to intervene directly in the vicinity of Taiwan."
AFPI was established in 2021 to promote Trump's public policy agenda. It involves a series of former senior officials from the Trump administration.