Pentagon planned to challenge 'illegal' hold on Ukraine aid before Trump phone call revealed
President Donald Trump takes a moment before taking the stage during a Memorial Day ceremony at Arlington National Cemetery in Arlington, Va., May 29, 2017. (DOD photo by U.S. Air Force Tech. Sgt. Brigitte N. Brantley)

The Pentagon was preparing a legal challenge to the White House scheme to block Ukraine aid in an apparent effort to force an investigation of Joe Biden.

Congress had approved hundreds of millions of dollars in aid to the U.S. ally in late 2018 as Ukraine fought against Russian aggression, and Pentagon officials began to worry as the funds unsent just weeks before the appropriation was set to expire Sept. 30, reported Yahoo News.

Pentagon officials were baffled that the funding hadn't been sent, even after John Rood, an undersecretary of defense for policy, gave Congress a detailed plan for what weapons and other aid Ukraine could expect from the package.

The Department of Defense conducted a series of interagency meetings starting in mid-July to figure out how the money could be freed up, and the Pentagon conducted a legal analysis of the holds.

That analysis determined the delays were illegal, but the Office of Management and Budget argued the holds were authorized by President Donald Trump -- which essentially made them legal.

“This is part of the basis for our investigation and overall impeachment inquiry,” one congressional staffer told Yahoo News.

Pentagon officials were not aware at the time that Trump was planning to use the congressionally approved aid package to pressure Ukraine's government to investigate the former vice president and his son Hunter Biden.

A call by Trump to Ukrainian president Volodymyr Zelensky discussing that scheme was the subject of a whistleblower complaint which has now turned into an impeachment inquiry.