In an unprecedented move, former United States Presidents Barack Obama and George W Bush delivered a heartfelt reprimand to the Donald Trump administration in an emotional farewell address to the staffers of the United States Agency for International Development (USAID). Obama termed the Trump administration's dismantling of USAID as "a colossal mistake."

Monday marked the end of an independent era for the six-decade-old humanitarian and development organization, established by President John F Kennedy as a peaceful means to promote US national security by fostering goodwill and prosperity abroad. Secretary of State Marco Rubio has ordered USAID to be absorbed into the US State Department starting Tuesday.

The former presidents and U2 singer Bono, who held back tears as he recited a poem, spoke with thousands in the USAID community in a videoconference, a closed-press event. They expressed their gratitude towards the thousands of USAID employees who have lost their jobs and their life's work. The agency was one of the first and most severely targeted for government cuts by Trump and his billionaire ally Elon Musk, with staffers finding themselves locked out of systems and offices and terminated by mass emailing.

Trump, in his defense, claimed the agency was run by "radical left lunatics" and rife with "tremendous fraud." Musk labeled it as "a criminal organization." Obama, in a recorded statement, offered comfort to the aid and development workers, some of whom were listening from overseas. "Your work has mattered and will matter for generations to come," he told them.

Obama has largely kept a low public profile during Trump's second term and has refrained from criticizing the seismic changes that Trump has made to US programs and priorities at home and abroad. "Gutting USAID is a travesty, and it's a tragedy. Because it's some of the most important work happening anywhere in the world," Obama said. He credited USAID with not only saving lives but being a major factor in global economic growth that has turned some aid-receiving countries into US markets and trade partners.

The former Democratic president predicted that "sooner or later, leaders on both sides of the aisle will realize how much you are needed." Asked for comment, the State Department said it would be introducing the department's foreign assistance successor to USAID, to be called America First, this week. "The new process will ensure there is proper oversight and that every tax dollar spent will help advance our national interests," the department said.

USAID oversaw programs around the world, providing water and life-saving food to millions uprooted by conflict in Sudan, Syria, Gaza, and elsewhere, sponsoring the "Green Revolution" that revolutionized modern agriculture and curbed starvation and famine. The agency worked at preventing disease outbreaks, promoting democracy, and providing financing and development that allowed countries and people to climb out of poverty.

Bush, who also spoke in a recorded message, went straight to the cuts in a landmark AIDS and HIV program started by his Republican administration and credited with saving 25 million lives around the world. Bipartisan blowback from Congress to cutting the popular President's Emergency Plan for AIDS Relief, known as PEPFAR, helped save significant funding for the program. But cuts and rule changes have reduced the number getting the life-saving care.

"You've shown the great strength of America through your work – and that is your good heart," Bush told USAID staffers. "Is it in our national interests that 25 million people who would have died now live? I think it is, and so do you," he said.

Bono, a longtime humanitarian advocate in Africa and elsewhere, was announced as the "surprise guest" and was wearing shades and a cap. He jokingly hailed the USAID staffers as "secret agents of international development" in acknowledgment of the nature of Monday's unofficial gathering of the USAID community. Bono recited a poem he had written to the agency about its gutting. He spoke of children dying of malnutrition, a reference to millions of people who Boston University researchers and other analysts say will die because of the US cuts to funding for health and other programs abroad.