Steve Bannon, an architect of Donald Trump’s 2016 election victory, was arrested on a yacht on Thursday and pleaded not guilty after being charged with defrauding donors in a scheme to help build the president’s signature wall along the US-Mexico border.

As a top adviser to Trump’s presidential campaign, who later served as White House chief strategist, Bannon (above) helped articulate the “America First” right-wing populism and fierce opposition to immigration that have been hallmarks of Trump’s three-and-a-half years in office. Trump fired Bannon from his White House post in August 2017...