In 2019, Waas broke numerous exclusive stories for "Vox" and "The New York Review of Books" about the impeachment investigation of President Trump. As explained by Waas in "Vox", "At the core of the impeachment inquiry is a substantial body of evidence that President Trump, both personally and through subordinates, pushed Ukraine to investigate former Vice President's Joe Biden's son, Hunter, and his business dealings in Ukraine. This pressure campaign stood to materially benefit Trump's 2020 presidential reelection effort by manufacturing dirt against a key rival. It is alleged that Trump withheld $390 million in congressionally-approved military assistance to Ukraine for months pending Zelensky's public agreement to open an investigation."