When North Korea launched a missile that flew over Japan on Friday morning, prompting 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 defences are limited, and the Constitution limits military action only to instances of self-defence.

In recent months, the government of Prime Minister Shinzo Abe has revived a long-simmering discussion over whether to acquire cruise missiles — which can be fired from land, air or sea — that would allow it to strike a launch site in North Korea if it detected signs of an imminent attack.

Itsunori Onodera, Japan’s Defence Minister, has avoided discussing a pre-emptive strike on North Korea. Instead, he speaks of counterstrikes, suggesting a more passive interpretation of the country’s legal rights under the Constitution.

To best protect itself from a missile attack, some experts say, Japan should buy a Terminal High Altitude Area Defense system, or THAAD, which intercepts enemy rockets at higher altitudes than its current land-based systems. The U.S. recently completed deploying THAAD in South Korea over vociferous protests from China, which has retaliated against the South by punishing it economically. That response has given some in Japan pause.

Instead, Japan has said it plans to equip and deploy more destroyers with the Aegis missile defence system. The Defence Ministry has also indicated it wants to acquire a land-based system, known as Aegis Ashore, which can intercept missiles above the atmosphere and above THAAD’s range.NYT