The launches have included at least two new types of short-range ballistic missiles and a mobile launcher that can fire multiple rockets. Pyongyang also has shown off a submarine that may be intended to carry nuclear warheads.

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Trump says he has been told that North Korean leader Kim Jong Un “only smiles when he sees me.” But photos released by North Korean state media show the dictator beaming from cheek to cheek at the successful tests.

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“There’s no question that the 2019 testing campaign that began in April has showcased some quite serious qualitative advancement in North Korean missile capabilities,” said Ankit Panda, an adjunct senior fellow in the Defense Posture Project at the Federation of American Scientists. “The core theme across all of the new weapons seems to be survivability, responsiveness and ­missile-defense defeat.”

The weapons that North Korea has showcased, including a road-mobile short-range ballistic missile known as the KN-23, with a range of at least 280 miles, appear designed specifically to confound South Korea’s missile-defense system.

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“The three missiles have several things in common: They are solid fuel, they are mobile, they are fast, they fly low, and at least the KN-23 can maneuver in-flight, which is very impressive,” said Vipin Narang, an associate professor of political science at the Massachusetts Institute of Technology.

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“Any one of the missiles would pose a challenge to regional and [South Korean] missile defenses given these characteristics. Together, they pose a nightmare.”

On Wednesday, South Korea’s Defense Ministry announced that it would raise defense spending by an average of more than 7 percent a year for the next five years, with money set aside to improve its radar detection and missile capabilities, to “secure ample interception capabilities against new types of ballistic missiles North Korea has recently test-fired.”

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South Korea’s missile-defense system was primarily built around the threat posed by North Korea’s older, comparatively clumsier Scud-class missiles. It includes U.S.-made mobile Patriot and PAC-3 missiles, the sea-based Aegis system and the land-based Terminal High Altitude Area Defense (THAAD) system.

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No system is impregnable, but North Korea’s new missiles appear designed to find one of the biggest gaps in Seoul’s armor.

Traditional ballistic missiles fly in an arc that takes them out of Earth’s atmosphere. But the KN-23, which appears similar to the Russian Iskander missile, took a lower trajectory, spending much of its flight at an altitude of 25 to 30 miles — potentially too high for the Patriot batteries, but too low for THAAD and Aegis systems to easily intercept.

Ferenc Dalnoki-Veress, scientist-in-residence at Middlebury Institute of International Studies at Monterey, Calif., said a ballistic missile flies in a predictable arc defined by gravity, just like a baseball thrown high into the air, making it easier to catch. The KN-23 is like a knuckleball — fast, low, unpredictable and almost impossible to catch.

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That the latest missiles are solid-fueled makes them easier to deploy and fire on short notice: Liquid fuel is corrosive and less stable, and it has to be added to a missile just before launch, a process that can give an adversary vital warning. Solid-fuel rockets, mounted as these have been on vehicles, can be hidden, moved around at will and launched quickly, making them almost impossible to take out before they are fired.

Although North Korea may not be able to miniaturize a nuclear warhead sufficiently to attach it to a missile such as the KN-23, conventional warheads that hit South Korean nuclear power plants could be devastating, experts say.

“I hope nuclear warheads will never be affixed to the KN-23, but if they are, it will be impossible for a threatened country to discriminate between an incoming nuke or high-explosive,” said Melissa Hanham, a missile expert at the One Earth Future Foundation. “This leads to a very destabilizing dynamic that will likely lead to escalation and preemptive action.”

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Finally, the fact that North Korea fired off 10 of the KN-23 missiles during the past four months shows it has no shortage of inventory, Narang said, suggesting that Kim has kept a promise made at the beginning of last year to move to a new phase of mass-producing missiles and nuclear bombs.

Saturday’s test appeared to show off a second type of short-range missile, which the state-run Korea Central News Agency described as a new weapon that has an “advantageous tactical character different to the existing weapon systems.”

Jeffrey Lewis, a scholar at the Middlebury Institute, said it was too soon to be sure about this new weapon but said it looked like a different class of short-range missile, similar in shape but larger than the U.S. Army Tactical Missile System or Israel’s Long Range Attack (LORA) missile.

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But the tests have not only been designed to raise North Korea’s military capabilities. They also have helped Kim bolster his reputation at home as a strongman determined to defend the regime’s security.

Kim may have come under domestic pressure after not winning many concrete benefits from his engagement with the United States and his moratorium on nuclear and intercontinental ballistic missile tests, experts say. This latest round of tests may have helped shore up that flank.

But the tests have had the added benefit of ramping up pressure on the United States to return to the negotiating table with a better offer than Trump presented in Hanoi in February. They can also help North Korea drive a diplomatic wedge between Washington and Seoul, by threatening South Korea without crossing any red line for Trump.

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North Korea has insisted that its launches are merely a response to joint U.S.-South Korean military exercises. It says Trump had promised to halt those exercises when he met Kim. But it has reserved its most bitter vitriol for the South Koreans, labeling them warmongers and pledging to exclude them from any future dialogue with the United States.