Now, with standing forces of about 5,000 to 10,000 troops, the Baltics feel vulnerable despite being members of NATO. They have no tanks, no air forces to speak of, and only patrol craft and minesweepers to ply coastal waters. Each country is now rushing to correct this shortfall.

The Estonians have a “defense league” that is made up of about 30,000 civilians and includes farmers, carpenters, lawyers and other professions. They engage in basic infantry training once a month, receive arms from the government, and in the event of an invasion would be called to active duty to be commanded by professional soldiers.

Juozas Olekas, Lithuania’s defense minister, said in an interview that the government was developing a more comprehensive self-defense plan coordinating across several government agencies. The army will soon add some 3,000 new conscripts.

In Latvia, Defense Minister Raimonds Vejonis said that with the Baltics’ bitter history under Soviet occupation, the public and the government were only too aware of Mr. Putin’s attempts to use propaganda and military might in Ukraine to intimidate NATO’s smallest members. “We will stay united because if we don’t, NATO will die,” said Mr. Vejonis, who will become Latvia’s president in July.

Not all of the NATO allies are as ardent. While there has been striking unanimity against Russia’s actions in Ukraine — separately, the European Union extended its sanctions against Russia this week — divisions remain.

“There’s a hope this is all a bump in the road and with a little bit of tweaking we can get back to the status quo,” the former American ambassador to Russia, Michael McFaul, said in a telephone interview. “In my view, that’s naïve. Putin’s not going to change his position, and he’s not going away. You’ve got to be in this for the long haul.”