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.
Though Japan provided rear support for the United States during the Vietnam and Korean Wars, this alliance has never been tested as it would be in a conflict with North Korea.
Any military action by the Trump administration against the North risks a retaliatory missile attack on Japan, where 54,000 American troops are based. On Friday, North Korea threatened to “sink” Japanese islands with nuclear weapons, adding that “Japan is no longer needed to exist near us.”
Japan’s position east of North Korea also means that missiles fired by the North toward the United States, including Guam, almost certainly would have to fly over Japanese territory.
But the missile defense systems stationed across Japan on mobile launchers are designed only to intercept missiles as they are descending, not in midflight as they are headed to the United States. Other defense systems on four naval destroyers can target missiles midflight, but they have to be in the right place at the right time.
It is also unclear whether the pacifist Constitution allows Japan to shoot down a missile headed for the United States, much less initiate a pre-emptive attack on a missile on a launchpad in North Korea, as some in Japan believe it should be prepared to do.
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.
The Japanese government ruled in 1956 that such a pre-emptive strike fell under its right of self-defense, but some lawmakers say deploying cruise missiles could cross a line and break with longstanding policy established after World War II to eschew offensive weapons. While the Japanese public is anxious about North Korea, it is torn about developing the nation’s military capabilities.
“The Japanese public is still not so sure about this,” said Richard Samuels, a Japan specialist and the director of the Center for International Studies at the Massachusetts Institute of Technology.
President Trump signaled this month that he wanted Japan, along with South Korea, to bolster arms spending. In a Twitter post two days after North Korea conducted its sixth nuclear test, Mr. Trump said he would allow the two countries to “buy a substantially increased amount of highly sophisticated military equipment from the United States.”
It is unclear whether Mr. Trump had specific equipment in mind, or whether that included cruise missiles.
In Japan, part of the political calculation is how China or South Korea might react to such a purchase. “It will be an excuse to China for further military buildup,” said Koji Murata, a professor of international relations at Doshisha University in Kyoto. “And even in South Korea, some kind of anti-Japanese sentiment will be further facilitated.”
Itsunori Onodera, Japan’s defense 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.
“In Japan’s case, I don’t think we can shoot before we are shot,” said Noboru Yamaguchi, a professor of international relations at the International University of Japan in Niigata and a retired lieutenant general in Japan’s army, known as the Ground Self-Defense Force. “Most likely, once we are shot and the second or third missiles are coming and they are on the ground, we can shoot back.”
Some analysts say that officials in Mr. Abe’s administration have been careful to use language that will not alarm the public. In polls, about half those surveyed say they would oppose Japan’s acquiring missiles to be used in pre-emptive strikes.
But as North Korea steps up missile launches and nuclear tests, Mr. Abe and his cabinet can make a stronger argument for such missiles. “They can say, ‘Look at what North Korea is doing. Yes, we have to protect ourselves,’” said Jeffrey W. Hornung, a political scientist at the RAND Corporation.
An upgrade of the country’s ballistic missile defenses would be a much easier sell politically.
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 United States 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 defense system. The Defense 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.
Still, most experts say that missile defense is hardly foolproof.
“Missile defense is still limited and very expensive, so you have to be somewhat lucky at this point,” said Patrick M. Cronin, senior director of the Asia-Pacific Security Program at the Center for a New American Security in Washington. “To have the political will to fire, you are taking a big gamble. Because if you miss, how does that look?”
North Korea has stated its clear intention to develop nuclear weapons capable of hitting the mainland United States and has threatened to strike the waters near Guam in the western Pacific with an “enveloping fire.”
Two years ago, Mr. Abe helped push through security legislation that authorized overseas combat missions by the Japanese military alongside allied troops in the name of “collective self-defense.”
For Japan to participate in such collective action, the new laws say, its own security must also be under threat. Some analysts question whether, under that definition, lawmakers would authorize an effort to shoot down missiles en route to the United States.