In a 2016 profile in the "Green Bay Press Gazette", Gallagher blamed President Barack Obama and former Secretary of State Hillary Clinton for the success of ISIS in Iraq. In 2019, he wrote it would be "a smart geopolitical move" for the U.S. to buy Greenland, a notion that Trump floated. In 2020, Gallagher voted against a measure to block Trump from taking military action against Iran without Congress's consent. In 2017, he supported a U.S. airstrike in Syria in retaliation for the Khan Shaykhun chemical attack, and in 2020 he supported the U.S. drone strike that targeted targeted Iranian general Qasem Soleimani. In 2019, Gallagher voted for a measure opposing Trump's decision to withdraw U.S. forces from Syria.