Flight-trackers spotted two U.S. military surveillance planes over the Korean Peninsula in early February 2020, fueling speculation that the Pentagon is expecting North Korea soon to test a new strategic weapon.

A U.S. Navy EP-3E flew over South Korea at an altitude of 25,000 feet, South Korean news agency Yonhap reported, citing Aircraft Spots, a popular Twitter account that monitors aircraft transponders.

A U.S. Air Force E-8C followed the EP-3 at 31,000 feet, Yonhap added. “The U.S. surveillance aircraft have often been detected here in recent months, apparently to intensify monitoring of the bellicose [North Korean] regime,” the news agency explained.

The EP-3E is a signals-intelligence aircraft that passively listens to emissions from radars and other systems. The E-8C, by contrast, is an active-sensor platform that uses its powerful underslung radar to map battlefields and track vehicles.

Both types are slow and vulnerable to enemy air-defenses, limiting their usefulness in high-intensity combat. But during peacetime they can fly with impunity inside friendly territory, surveilling enemy forces from a distance.

The Air Force is in the process of replacing the 16 aging E-8s with new systems, potentially including sensors on drones and stealth fighters. The Navy hasn’t decided on a replacement for its 11 EP-3s.

The overflights come as North Korea could resume testing of a nuclear-capable intercontinental ballistic missile. Pyongyang in late 2019 hinted at the test, describing it as a “Christmas surprise” for the United States.

Christmas came and went without any major missile launch. But the Pentagon remains on alert for a possible test.

A fresh test of an ICBM, the first since 2017, would signal a significant escalation of Pyongyang’s efforts to build up a nuclear arsenal capable of directly threatening the continental United States.

The test also would signal a deepening failure of U.S. president Donald Trump’s on-again, off-again campaign to forge some kind of deal with North Korean leader Kim Jong Un.

As a demonstration of goodwill toward Trump, North Korea halted ICBM testing following a successful trial flight in 2017.

But three summits between Trump and Kim, starting with a historic first meeting in Singapore in June 2018, failed to produce any sort of deal that satisfied America’s both goal of denuclearizing the Korean Peninsula and North Korea’s goal of legitimizing its new nuclear deterrent while also loosening international economic sanctions.

Talks stalled following a widely-publicized June 2019 handshake between Trump and Kim on the North Korean side of the demilitarized zone separating the two Koreas. Amid this impasse, North Korea's foreign ministry has openly insulted Trump, accusing him of lapsing into the "the dotage of a dotard."

And in December 2019 Pyongyang began hinting at the “Christmas surprise” in the event diplomacy didn’t get back on track.

The Pentagon was clear about what it believed that surprise to be. “What I would expect is some kind of long-range ballistic missile would be the ‘gift,’” U.S. Air Force general Charles Brown, head of Pacific Air Forces, told reporters on Dec. 17, 2019. “Does it come on Christmas Eve? Does it come on Christmas Day? Does it come after the new year? One of my responsibilities is to pay attention to that.”

“Brown declined to say whether the U.S. military has noticed an uptick of movement around North Korean launch sites or if it had any indications of what kind of missile could be used,” Defense News reported. “However, he noted that North Korea has already publicly acknowledged test activity at launch sites in recent days.”

One of those tests took place on Dec. 14, 2019, the state-owned Korean Central News Agency announced. That “crucial” test is a step toward "reliably restraining and overpowering the nuclear threat of the U.S.," Pak Jong Chon, chief of the general staff of the Korean People’s Army, told KCNA.

As a new ICBM test seemingly grew imminent, American diplomats grew desperate. "Let me speak directly to our counterparts in North Korea," Stephen Biegun, the U.S. special envoy for North Korea, said in December 2019. "It is time for us to do our jobs. Let's get this done. We are here, and you know how to reach us."

Biegun conceded it’s been "a long year and we have not made nearly as much progress as we would have hoped."

“We will not give up,” he added.

Trump at one point staked his perhaps-undeserved reputation as a deal-maker on preventing a North Korean ICBM test. “North Korea just stated that it is in the final stages of developing a nuclear weapon capable of reaching parts of the U.S.,” Trump tweeted in January 2017. “It won't happen!”

But it did happen. Pyongyang conducted no fewer than three ICBM tests before voluntarily pausing trials. If North Korea does resume testing, it’s unclear exactly what this will mean for the country’s deterrent and the prospects of lasting peace.

David Axe serves as Defense Editor of the National Interest. He is the author of the graphic novels  War Fix, War Is Boring and Machete Squad.