Such vows of retribution may be common in the West when leaders face extremist violence, but they have been unheard of in confrontation-averse Japan — until now. The prime minister’s call for revenge after the killings of the journalist, Kenji Goto, and another hostage, Haruna Yukawa, raised eyebrows even in the military establishment, adding to a growing awareness here that the crisis could be a watershed for this long pacifist country.

“Japan has not seen this Western-style expression in its diplomacy before,” Akihisa Nagashima, a former vice minister of defense, wrote on Twitter. “Does he intend to give Japan the capability to back up his words?”…

“This is 9/11 for Japan,” said Kunihiko Miyake, a former high-ranking Japanese diplomat who has advised Mr. Abe on foreign affairs. “It is time for Japan to stop daydreaming that its good will and noble intentions would be enough to shield it from the dangerous world out there. Americans have faced this harsh reality, the French have faced it, and now we are, too.”