The foreign policy expert, who served under president Jimmy Carter, told the Senate Armed Services Committee that he feared Russian President Vladimir Putin might try to take control over Baltic countries in a lightning move that could take NATO by surprise.
A nightmare scenario could be that "one day - and I literally mean one day - he just seizes Riga and Tallinn... That would literally take him one day. There's no way they could resist," Brzezinski said.
"And then we'll say how horrible, how shocking, how outrageous. But, of course, we can't do anything about it," he said, without risking a potential nuclear conflict.
"I think deterrence has to have meaning. It has to have teeth in it. And it has to create a situation in which someone planning an action like that has no choice but to anticipate what kind of resistance will lie in counter," he said.
"I do recommend pre-positioning of some forces," in those countries, he said, but in a way that was not provocative.
Brzezinski served as the national security adviser under Jimmy Carter from 1977 to 1981 and is a professor at Johns Hopkins University's School of Advanced International Studies.