Iraq War veteran Chris Sheppard fumed as he watched President Trump's joint news conference with Russian President Vladimir Putin in Helsinki.

The former combat engineer with the U.S. Marine Corps sat glued to his cellphone screen in his downtown Seattle office, watching live on Monday as the American president suggested he believed Putin's denial that his agents interfered in the 2016 U.S. elections. Trump also declined to say whether he believed the U.S. intelligence community's conclusion that Russia interfered.

Sheppard, who left the military after 13 years in 2005 and is now a tax attorne...