The U.S. and allies flew bombers over the Korean Peninsula in a show of force. Here’s why that’s important.

American B-1B bombers and F-35s, along with Korean and Japanese military aircraft, overflew the Korean Peninsula early Thursday morning in what military leaders characterized as a direct response to North Korea’s launch of a ballistic missile over Japan earlier this week.

“North Korea’s actions are a threat to our allies, partners and homeland, and their destabilizing actions will be met accordingly,” said Gen. Terrence J. O’Shaughnessy, commander of U.S. Pacific Air Forces. The mission, which was not part of the U.S.-South Korea exercises that ended today, also included South Korea Air Force F-15K fighters and F-15Js from Japan.

Why this demonstration? Thursday’s mission, which sent 12 aircraft from Kyushu, Japan, over the Korean Peninsula to release live weapons over a military training range, was “intended as a signal to North Korea of allied determination to deter, defend, and defeat [North Korean] attacks,” said Bruce Klingner, a senior research fellow Northeast Asia at the Heritage Foundation. For U.S. allies, the exercises also underscored America’s commitment to defending them, Klingner said.

They were also as much “a signal to North Korea of the allies’ capabilities” as it was a reassurance of the allies’ defense, and their “resolve to respond to NK’s aggressions swiftly, and with strength,” said Balbina Hwang, who served as a special State Department advisor on East Asia and Pacific issues before becoming a lecturer at Georgetown and American universities.

The other significance: The exercises “included Japanese air defense forces, and the inclusion of Japan as part of an allied active defense of the ROK cannot be underestimated.”

Is this unusual? The U.S. has flown B-1Bs over the Korean Peninsula often, including early this month, but it’s the first time that F-35s, America’s newest stealth planes, have accompanied the B-1Bs.

The mission also ended with a drop of live weapons, which is rare for these kinds of demonstrations, officials say.

READ MORE: What we know about the missile North Korea fired over Japan

President Barack Obama rarely ordered immediate responses to North Korean missile tests, “partly because he believed it gave DPRK the attention they were seeking,” said Frank Jannuzi, president and CEO of the Mansfield Foundation and a longtime federal analyst who was part of the team that negotiated the Agreed Framework with North Korea during the Clinton administration.

But there have been far more missile tests since Kim Jong Un took office in May 2016 — more than 80 — than there were under his father or grandfather’s rule, Jannuzi said.

This is unlikely to affect North Korea, says Joel S. Wit, a senior fellow at the U.S.-Korea Institute at SAIS and senior research fellow at the Columbia University Weatherhead Institute for East Asian Studies. “It’s like groundhog day. They do a test, we fly bombers … and sometimes others join us,” he said.

But it is “an important signal of growing regional and allied cooperation to find a coordinated response,” Hwang said. Cooperation between South Korea and Japan on security issues “has been politically challenging, if not impossible at times, and certainly many expected that it would become even more difficult under South Korean President Moon,” Hwang said. These exercises indicated a willingness to work together in the face of a North Korean threat.

The thing to watch: whether Kim Jong Un will order a nuclear test, Jannuzi says. North Korea appears to be poised to conduct a test, based on what Jannuzi has seen from unclassified satellite data. The only reasons for North Korea to hold off: threat of more severe sanctions, or, a sign that dialogue could lead to sanctions relief, Januzzi says.

READ MORE: Analysis: Should the U.S. set a ‘red line’ for North Korea?