Nuclear security

Russia reportedly warned Mattis it could use nuclear weapons in Europe, and it made him see Moscow as an 'existential threat' to the US

At some point during the Trump administration, Russia told Defense Secretary Jim Mattis that it could use nuclear weapons in the event of a war in Europe — a warning that led Mattis to regard Moscow as major threat to the US.

According to "Fear," Bob Woodward's recently released book about turmoil in the White House, Moscow's warning was in regard to a potential conflict in the Baltic countries of Estonia, Latvia, and Lithuania.

The Baltics were part of the Soviet Union and have deep ties to Russia, which has sought to reassert influence there since the end of the Cold War. Those countries have tried to move closer to the West, including NATO membership.

According to Woodward's account, the warning from Russia came some time during or before summer 2017, when the Trump administration was haggling over the future of the Iran nuclear deal.