The forcing down of a U.S. drone by Russian fighter jets over the Black Sea on Tuesday spiraled into a diplomatic incident and fueled concern about the potential for the war in Ukraine to escalate into direct conflict between the two superpowers.

The incident also shone light on a fact of geopolitics that often receives little attention: Close calls, some harrowing, between U.S. and Russian aircraft aren't unusual.

The United States and NATO allies often intercept Russian jets flying close to Alaska or to NATO airspace above the Black or Baltic seas. Russia also has intercepted American aircraft in those regions, sometimes swooping close enough to cause turbulence.

This week's incident "follows a pattern of dangerous actions by Russian pilots while interacting with U.S. and allied aircraft over international airspace," U.S. Air Force Gen. James B. Hecker, commander of the U.S. Air Force in Europe and Africa, said in a statement Tuesday.

Crashes caused during intercepts are extremely rare, however, even for drones. Analysts and officials warn that the war in Ukraine has only heightened the stakes.

The U.S. military released a video Thursday of what it says is a Russian fighter jet clipping a U.S. surveillance drone. The United States said the resulting damage forced it to bring the drone down into the Black Sea. Moscow has denied that either fighter hit the drone, and blamed the United States for flying the craft too close to territory in Crimea that Russia claims to have annexed.

Here's what to know about the history of close encounters between Russia and the United States in the skies.

- - -

How did a U.S. drone end up in the Black Sea?

Washington and Moscow have given conflicting accounts for how, exactly, a U.S. MQ-9 Reaper drone ended up crashing.

U.S. officials said the drone was flying in international airspace about 50 miles off the coast of Crimea - the Ukrainian peninsula Russia seized in 2014 - when two Russian Su-27 fighter jets buzzed it. The footage shared Thursday appears to show two separate passes by the jets, which sprayed a cloudy white stream. U.S. officials said the Su-27s dumped fuel on the drone before one of them collided with its rear propeller, prompting U.S. personnel operating the craft remotely to bring it down in international waters. In the video released by the U.S. military, the propeller stops spinning and a chunk of it appears to be missing after the second pass by the Russian fighter.

Before the release of the video, Russia claimed no collision occurred between the aircraft, chalking up the crash to "sharp maneuvering" by the drone, which it said was flying with its transponder turned off. Moscow blamed the United States for breaching a "temporary" boundary delineated by Moscow for its "special military operation" in Ukraine. Russian officials had no immediate reaction to the release of the video on Thursday.

Territorial waters extend 12 miles from the shore under international law - and the United States and allies reject both Moscow's annexation of Crimea and its claim that it can unilaterally box off international airspace for its own purposes because of its invasion of Ukraine.

- - -

Is this sort of incident unusual?

Aircraft intercepts - when military aircraft approach another aircraft to evaluate it or pressure it to change course - occur frequently. When they don't result in incident or disaster, they rarely attract broad attention.

Before Russia launched its full-scale invasion of Ukraine last year, NATO planes were involved in an annual average of 400 intercepts with Russian planes, the Associated Press reported.

Both sides initiate these encounters. Last month, the United States intercepted Russian aircraft flying in international airspace near Alaska on two consecutive days. Earlier in February, U.S. F-15Es scrambled and intercepted Russian fighter jets near NATO airspace over the Baltic Sea.

NATO allies also intercept Russian planes. In the latest example, the U.K. defense ministry said fighter jets from Britain and Germany, which are helping Estonia to police its airspace, intercepted a Russian refueling aircraft close to Estonian airspace after it failed to communicate with Estonian air traffic control.

The United States, meanwhile, often flies reconnaissance flights in international airspace on Russia's periphery, and "there have been multiple cases of the use of very aggressive intercepts" by Russia, said Samuel Charap, an analyst at the RAND Corporation.

"This fits a pattern that dates to the '50s," he said. But in the post-Cold War period - especially after 2012 - Russia has carried out "more aggressive intercepts."

These typically involve Russian fighter jets approaching a U.S. or NATO aircraft, forcing it to change course. This has involved risky maneuvers and near misses, such as in August 2020, when Russian Su-27s intercepted a B-52 bomber taking part in what the U.S. Air Force called "routing operations" in international waters in the Black Sea, according to a 2021 Rand Corp. report. Russian jets crossed within 100 feet of the bomber's nose several times, the Air Force said.

In this week's incident, "what was different was the actual collision" the U.S. military says occurred, Charap said.

"We don't have cases of that," he said, adding that he's never observed Russian jets deliberately dumping fuel onto another aircraft.

White House spokesman John Kirby called the intercept "noteworthy because of how unsafe [and] indeed reckless" the Russian planes were.

It marked the first known altercation between U.S. and Russian forces since the war in Ukraine began in February 2022.

- - -

Why does Russia intercept U.S. aircraft?

It can be difficult to pinpoint Russian military strategy. But in their paper, Charap and colleagues argue that aggressive air intercepts are examples of "coercive signaling" from Moscow, "to send targeted messages regarding activities that it finds problematic."

The close encounters usually come in response to the presence of U.S. or allied ships or aircraft in areas that are militarily sensitive to Moscow. "Where Russia sees core interests potentially under threat, it might pursue more-assertive and potentially dangerous signaling efforts," the researchers argue.

The war in Ukraine has increased the risks from this aerial brinkmanship. While the United States has provided significant military and intelligence support to Ukraine - including aerial surveillance with drones - it has sought to avoid being drawn into a direct military confrontation with Russia.

The United States and Russia set up a phone line years ago to stave off collisions that could prompt a crisis. Close encounters in the airspace on Russia's periphery "dropped off significantly" in the first six months after Russia invaded Ukraine, Charap said.

U.S. officials have portrayed the incident Tuesday as the result of reckless, amateurish flying - and the clipping of the drone propeller as a possible accident - rather than part of a broader campaign against U.S. reconnaissance in the Black Sea. But they warned that dangerous actions could lead to "miscalculation and unintended escalation."

Philip Breedlove, a retired Air Force general and former supreme allied commander of NATO, said Tuesday that it's possible the incident was a result of a "stupid mistake."

But if the Russian jet deliberately collided into the American drone, "that would be concerning" and could indicate Moscow was "trying to change the narrative" by striking a U.S. asset.

The unmanned nature of the Reaper might have emboldened both sides, Charap said. If the life of a pilot were at risk, "the pressure to do something in response would be much more significant."

- - -

The Washington Post's Dan Lamothe, Karen DeYoung and Anumita Kaur contributed to this report.