President Trump’s incoming national security adviser suggested during the 2016 Republican primary campaign that Texas Sen. Ted Cruz attack then-candidate Donald Trump's attitude toward Russian President Vladimir Putin.

“Cruz will need to abandon his ‘kid glove’ treatment of Trump,” Robert C. O’Brien, the new White House national security adviser, wrote in a 2015 column. “He certainly has an opening to do so on national security versus Trump, who has been playing up how chummy he will be with Vladimir Putin if he is elected.”

That suggestion called attention to one of Trump’s perceived political vulnerabilities as a national security neophyte who had erred by exchanging compliments with Putin. Cruz, in the event, rebuked the debate moderators for trying to orchestrate “a cage match” between various presidential contenders. That strategy allowed Cruz to finish second overall in the primary campaign, but Trump of course went on to win the White House.

“Donald Trump’s foreign policy positions are not entirely clear, but he has centered his campaign around a Reaganesque promise to ‘Make America Great Again,’” O’Brien wrote in the epilogue of While America Slept, a collection of essays published in 2016. “America faces a stark choice in 2016 between a continuation of President Obama’s ‘lead from behind’ foreign policy and sequester based national security approach and a return to President Reagan’s ‘leader of the free world’ foreign policy and ‘peace through strength’ national security approach.”

O’Brien defended the transatlantic alliance that the GOP nominee had suggested might be “obsolete,” but he also affirmed the president’s call for increased defense spending by European allies. “NATO is the most successful military alliance in history,” he wrote. “Under American leadership, our NATO allies must be encouraged to invest in their own defense.”

The column was written months before the Russian cyberattacks against the Democratic Party that would give rise to the FBI investigation into suspected links between Trump and the Russian government. That two-year probe ended when special counsel Robert Mueller announced that he “did not establish” a criminal conspiracy with Russia. But the column, like the broader book, reflected O’Brien’s perspective as a conservative foreign policy expert with a hawk’s eye on the Kremlin.

“Russia's annexation of the Crimea and state-sponsored civil war in Ukraine is well known,” O’Brien wrote. "Its carve-up of Georgia and Moldova has been less widely reported. Success in those adventures may only encourage Moscow to attempt to reassert control or ‘Finlandize’ the Baltic States, all three of which are in NATO.”

O’Brien eventually joined the Trump administration as the special presidential envoy for hostage affairs in May of 2018. “He is a loyal servant of the president who has kept himself out of the spotlight and always made his work about the president and the Americans who he was saving,” an administration official, speaking on condition of anonymity, told the Washington Examiner.