The first prototype unit in this new force, an “interim spearhead force” made up of 3,000 to 4,000 German, Dutch and Norwegian troops, will be operational by next year, NATO officials said this month, but a permanent force will not be up and running until at least 2016.

Lithuania, however, decided it could not wait that long.

“The situation in the region has changed,” General Tamosaitis said. “And we need to counter these emerging threats, this new kind of hybrid war.”

Such conflict continues to simmer in eastern Ukraine, with Western leaders charging — and Moscow denying — that Russian troops are training, arming and fighting alongside local insurgents. And Russia has been increasingly provocative in its use of military flights and naval operations in the Baltic Sea.

Last Friday, the Danish aviation authorities had to warn a Swedish passenger jet leaving Copenhagen to change course to avoid a Russian military aircraft flying with its transponder turned off. Two days earlier, a plane flying out of Finland had a similar episode involving a Russian jet with its transponder off. In October, the region was transfixed for more than a week by the search for a submarine, suspected of being from Russia, which witnesses claimed to have spotted near the Swedish coast.

In all, NATO officials said early this month, there had been more than 400 incidents this year in which alliance aircraft were scrambled to match the presence of Russian military jets. That was an increase of 50 percent over 2013, they said.

“The Russians are testing what they can get away with,” Juozas Olekas, Lithuania’s defense minister, said in an interview. “They’re testing how we react. They are exercising. They are demonstrating their power.”