This recent overflight of Japan wasn’t North Korea’s first: In 1998, 2009, 2012, and 2016, North Korea overflew its territory with projectiles. None were designed to serve as strategic nuclear-delivery systems; all were satellite-launch vehicles, designed to deliver payloads into low-earth orbit and not reenter the atmosphere. Some analysts had long worried that the underlying technologies of these vehicles would eventually form the basis of an ICBM, but the large and unwieldy designs Pyongyang showed off had major shortcomings. For one, none of those designs would allow for the kind of mobility that North Korea’s much smaller KN17 and KN20 systems enjoy. (Without mobility, the missiles would be sitting ducks at fixed launch sites during wartime—an invitation to preemption by the United States.)

Nonetheless, the first of those launches in 1998 was regarded as an exceptional provocation, giving Japan a glimpse of a terrifying future in which North Korean missile overflies could become a fact of life. The launch sparked Japan’s ongoing interest in ballistic-missile defense, spurring investments in sea-based interceptors like the Standard Missile-3 Aegis interceptors and the Patriot Advanced Capability-3 system as a hedge against that possible future. (Only the former has the capability to intercept a missile like the one North Korea just flew over Japan, and only under particular conditions.)

As of this week, Japan’s terrifying future has merged with its present. Even as a clearly agitated Japanese Prime Minister Shinzo Abe reacted to North Korea’s launch on Tuesday morning, saying that his country would take “all possible measures” to ensure the safety of its people, he no doubt knew what lay ahead for Japan.

Earlier this year, Japan began ballistic-missile-attack evacuation drills for its citizens living near likely North Korean target areas. The Japanese government is also seeking earmarked funds in its 2018 budget request for new ballistic missile defense systems. Tokyo may also look to acquire the Terminal High Altitude Area Defense system—the defensive system that has been at the center of domestic and international controversy in South Korea—though this may be less cost effective. Finally, the inexorable growth of the North Korean threat toward Japan may well accelerate plans in Abe's right-wing Liberal Democratic Party to revise the country’s pacifist constitution, allowing Tokyo to formally wield precision-strike weapons for potential use against North Korea.

Even as Tokyo reckons with the arrival of its long-feared nightmare, it’s important to recognize that there are important reasons for Pyongyang—which remains rational despite the metronymic exhortations of many analysts to the contrary—to continue to overfly Japan with its missiles.

On a technical level, a ballistic missile flown at the kind of trajectory North Korea demonstrated this week experiences physical and temperature stresses more in line with what it would see during operational use. North Korea's “lofted” tests, which fly the missile to immense altitudes, keeping its range contained to the Sea of Japan, provide some useful data in this regard. However, a launch like this week’s gives North Korean scientists a chance to observe how the missile may perform in a real attack. (There are outstanding questions, though, about how North Korea would have gathered telemetry data from this missile in the northern Pacific Ocean, where it is thought to have splashed down.)