TOKYO — When North Korea launched a missile that flew over Japan on Friday morning, prompting the authorities to broadcast an alert on cellphones and television, many people wondered: Why didn’t the Japanese military shoot it down?

The government quickly judged that the missile was not targeting Japan, and it landed in the Pacific Ocean, about 1,370 miles east of Hokkaido, Japan’s northernmost island.

But officials in Japan who may have considered intercepting the missile faced two immediate constraints: The country’s missile defenses are limited, and the Constitution limits military action only to instances of self-defense.

Those same constraints have weighed heavily on the debate in recent weeks over how Japan should be responding to the North’s rapidly advancing nuclear program, including what role it should play as an American ally and to what extent it should upgrade its armed forces.