Public health experts say U.S. withdrawal from the W.H.O. would undermine the nation’s standing as a global health leader and make it harder to fight the next pandemic.

President Donald Trump used one of the flurry of executive actions that he issued on his first day back in the White House to begin the process of withdrawing the U.S. from the World Health Organization for the second time in less than five years — a move many scientists fear could roll back decadeslong gains made in fighting