Work spoke at the Norwegian-American Defense Conference at the National Press Club, where he hailed Norway's commitment to global peace and security, U.S. Department of Defense reports.
Europe and, by extension, NATO, faces challenges from 360 degrees, the deputy secretary said. To the east, west and north, it faces a "resurgent and apparently antagonistic Russia," while from the south and from within, terrorism and violent extremism are concerns.
Read alsoPentagon: Russia top global threat to U.S.Migration along Europe's southern flank is a daunting challenge caused by terrorism and violent extremism, he said.
"We feel that we are at a strategic inflection point in the strategic landscape," Work told the forum. "The next 25 years in the international security environment is going to be much, much different than the last 25."
A "revanchist and a resurgent Russia" is "pursuing a path of confrontation, aggression and coercion," Work said.
In 2012, the United States thought it was on a "path to partnership" with Russia, and reduced its last two heavy brigades from Europe, he said. The U.S. "believed and wanted to have a productive partnership with Russia into the future," Work added.
However, Russia in March 2014 illegally annexed Crimea and started to destabilize eastern Ukraine, he said. Russia has engaged in "nuclear saber rattling," Work said, adding that it has made threats against NATO allies, most recently Romania and Poland for hosting ballistic missile defense sites.
By 2017, the United States will have a full military division back in Europe, Work said. The United States, he said, is "exercising muscles" it hasn't used since the Cold War.
He said the United States felt it was important to be a deterrent and reassure its European allies it would be there to respond to Russia if needed.
"Even as we pursue the defeat of ISIL, and other violent extremist groups, we must also vigorously respond to the challenge in the east – Russia," he said.
He pointed out "reckless" behaviors from Russia, including "unsafe and unprofessional maneuvers" by Russian military aircraft and naval vessels.
The secretary highlighted the strong ties between U.S. and Norway. He noted he visited Norway last year, and President Barack Obama hosted Nordic leaders last week at a summit in Washington. The Marine Corps also has a pre-positioning program in Norway, he pointed out.
