Hypersonic missiles are a "nightmare" weapon to which the US is especially vulnerable, Maine Sen. Angus King warned the nominee to take over as chief of US naval operations.

King said the US is without defense for the new weapon, which Russia and China are developing, and cited the Battle of Agincourt in 1415 to illustrate the dangers of falling behind technologically.

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Hypersonic missiles have been a favorite topic of defense officials and legislators in recent years, but Sen. Angus King cautioned Wednesday that the United States is drastically underestimating the threat posed by these "nightmare" weapons.

King's warning came during the confirmation hearing for Vice Adm. Michael Gilday, director of the joint staff and President Trump's pick to take over as the chief of naval operations, before the Senate Armed Services Committee.

"I believe that this is a place where we have really fallen short," the independent Maine senator said. "And I'm not talking to you particularly. I'm talking about the entire defense establishment."

To illustrate his point, King posed a hypothetical situation to Gilday, asking him to imagine himself as the commander of the new USS Gerald R. Ford aircraft carrier, out on patrol off the eastern coast of Iceland.

The USS Gerald R. Ford underway on its own power for the first time, in Newport News, Virginia, April 8, 2017. US Department of Defense

"It's early in the morning, and you learn that a hypersonic missile has just been launched from Murmansk. You have 13 to 15 minutes to react," King said. "The missile is traveling at 6,000 miles an hour. It creates a plasma shield so that radar is useless. The only sensor is infrared, and we have inadequate infrared overhead architecture. What do you do?"

"Defend myself as best I can, sir," Gilday replied.

Hypersonic missiles are a new breed of weapon capable of flying at several times the speed of sound. Not only are they several times faster than traditional ballistic missiles, their maneuverability makes them extremely difficult to intercept.

"But when you're talking about something coming at 6,000 miles an hour that you can't see, that could have a nuclear warhead, that's maneuverable — that's a nightmare weapon," King said.

National security experts and defense officials have warned that the United States is drastically behind adversaries such as Russia and China in the development of hypersonics and potential countermeasures.

Michael Griffin, the undersecretary of defense for research and engineering, testified to Congress last year that the Pentagon does not "have systems which can hold [China and Russia] at risk in a corresponding manner, and we don't have defenses against [their] systems."

In King's opinion, hypersonics are a game-changer in modern warfare. He compared US aircraft carriers to the armor-clad French warriors who were defeated by a smaller force of English longbowmen in the early 15th century.

"Admiral, I would suggest that this is a disruptive change in the history of warfare," King said. "At the Battle of Agincourt in 1415, there were four times as many French as there were English. The English deployed for one of the first times the longbow. It changed the course of European history. This is a similar change, I view."

A Russian Air Force MiG-31K jet with a high-precision hypersonic aero-ballistic missile Kh-47M2 Kinzhal over Moscow, May 9, 2018. AP Photo/Alexander Zemlianichenko

The Battle of Agincourt, immortalized in Shakespeare's play "Henry V," was one of the most consequential conflicts of the Hundred Years' War between England and France.

The French deployed approximately 36,000 troops, many clad in the heavy armor popular at the time, versus the 6,000 or so English troops. Approximately 80% of the English force was lightly armored and wielded longbows, a larger version of a typical bow that had vastly superior range and firepower. The agile longbowmen devastated the cumbersome French force, securing a crucial English victory.

King argued the only way to counter a weapon like a hypersonic missile is to deter it, but he noted the United States isn't expected to field a hypersonic weapon until the mid-2020s. The Russians, meanwhile, are expected to field the Zircon hypersonic missile as early as next year.

"The Chinese are very close behind," King said. "We are woefully inadequate both in terms of developing this capability ourselves but more particularly in coping with it."