A pair of the U.S. Air Force’s best surveillance planes on Monday flew over eastern Ukraine fewer than 40 miles from territory controlled by Russian-backed separatists.

The two-jet mission, involving an E-8C ground-surveillance plane and an RC-135V signals-intelligence plane, is one of the first missions that the USAF openly has flown inside Ukrainian air space—and comes nine months after Russia built up a potential invasion force along the border with Ukraine.

That force—including around 100,000 troops, 1,200 tanks and scores of specialized vehicles—has diminished slightly in recent days as 10,000 troops wrapped up what Russian officials characterized as a long-planned exercise, and returned to their bases.

But more than enough of Moscow’s forces remain in position for an attack. The U.S. State Department in mid-December warned Americans against traveling to Ukraine. “U.S. citizens should be aware of reports that Russia is planning for significant military action against Ukraine.”

A diplomatic resolution still is possible. Following an early December phone call between U.S. president Joe Biden and Russian president Vladimir Putin, in which Biden warned Putin of potential economic sanctions if Russia invades, the U.S. and Russian governments agreed to talks in January.

In the meantime, Washington has boosted its support for Kiev. The United States has transferred to Ukraine billions of dollars worth of radars, patrol boats and anti-tank missiles. A U.S. military team recently traveled to Ukraine to assess the country’s air-defense network. And now the Americans are gathering intelligence over Ukraine—and presumably sharing it with the Ukrainians.

The E-8C and RC-135V make a compelling team. The E-8C packs an underslung radar with so-called “ground moving target indicator,” or GMTI, capability—meaning it repeatedly can scan for vehicles over time, giving operators a sense of the vehicles’ direction of travel and speed.

The RC-135V is a signals-intelligence, or SIGINT, plane whose sensitive receivers pick up enemy transmissions. An E-8 and RC-135 working together could “cross-cue.” The former detects tanks and other vehicles. The latter logs their communications. A few passes by the duo could give commanders a comprehensive view of Russian forces.

There’s risk. Russian foreign minister Sergey Lavrov has warned the Biden administration that “the deployment of NATO forces in the immediate vicinity of our borders will have the most serious consequences.”

The threat didn’t deter the White House. E-8s and RC-135s generally are visible to the public only when they fly with their transponders on, meaning they appear on any of several flight-tracking websites. The USAF could have ordered the crews to turn off their transponders for the Monday mission.

It didn’t. The mission was more than an effort to surveil Russian forces. It was a statement from the Biden administration to the Putin regime.

The Monday sorties also underscored the sophistication, and fragility, of the U.S. military’s surveillance apparatus. Few countries possess the same mix of GMTI and SIGINT capabilities. The Royal Air Force once did, until it decommissioned its Sentinel radar planes—its main ground-scanning aircraft—and sold them to the U.S. Army.

But the USAF for one is about to give up most of its GMTI fleet. Having decided that the big, slow E-8s can’t survive near enemy air-defenses in wartime, the Air Force gradually is retiring the type. The service in 2022 plans to divest four of its 12 remaining E-8s.

While the Army is standing up its own ground-scanning force, the Air Force’s is set to dwindle until just 10 GMTI-equipped Block 40 Global Hawk drones remain. Monday’s cross-cueing surveillance mission over Ukraine was significant. It might get harder in coming years for the Pentagon to duplicate the feat.