"We are engaged, as we speak, to ensure alliance is relevant and fit for its time. The United States is moving units into the Baltic states – Poland, Romania, and Bulgaria – under Operation Atlantic Resolve," Mattis said. "This is a wholly defensive deployment to maintain deterrence."
NATO also made a decision to send multinational battalions to Latvia, Lithuania and Estonia at its Warsaw summit in July.
The alliance has been increasing its military presence in the Eastern Europe since 2014, using vulnerability to alleged Russian aggression as a pretext. Russia has repeatedly voiced its protest against the increased number of NATO troops near its borders.