KALISZ, Poland — For evidence of how much President Vladimir V. Putin of Russia has jangled nerves and provoked anxiety across Eastern Europe, look no farther than the drill held the other day by the Shooters Association. The paramilitary group, like more than 100 others in Poland, has experienced a sharp spike in membership since Mr. Putin’s forces began meddling in neighboring Ukraine last year. Thirty students took an oath to defend Poland at all costs, joining nearly 200 other regional members of the association — young men and women, boys and girls — marching in formation around the perimeter...