Russia has worked to enhance its naval capabilities since 2000.
The war in Syria has given it a chance to test those new assets, and Western and Russian warships now both operate in the eastern Mediterranean.
Their operations there underscore how naval tactics and strategy are shifting.
Western and Russian warships have been in close proximity in the eastern Mediterranean, where both sides are assisting partners fighting in Syria.
Both sides have used it as an opportunity to keep tabs on each other, studying their adversary's capabilities and tactics.
A Russian attack submarine left the Baltic Sea in early May, heading to the eastern Mediterranean, according to The Wall Street Journal.
It was tracked along the way by NATO ships, including by a Dutch frigate that took a photo of the sub in the North Sea.
Russian Krasnodar, as spotted by a Dutch frigate in the North Sea. (Image Adm. Rob Kramer Twitter)
By the end of the month, it had arrived on station, and the Russian Defense Ministry announced the cruise missiles it fired hit ISIS targets near Palmyra in Syria.
A few days later, the USS George H.W. Bush sailed through the Suez Canal, meant to support US-backed rebels in Syria.
For sailors and pilots from the Bush, with little formal training in anti-sub operations, their duties now included monitoring the Krasnodar.
"It is an indication of the changing dynamic in the world that a skill set, maybe we didn't spend a lot of time on in the last 15 years, is coming back," Capt. Jim McCall, commander of the air wing on the USS Bush, told The Journal.
The cat-and-mouse game continued in the eastern Mediterranean throughout the summer.
US helicopters ran numerous operations in search of the sub. Flight trackers also picked up US aircraft doing what seemed to be anti-submarine patrols off the Syrian coast and south of Cyprus. In mid-June, the Krasnodar fired more cruise missiles at ISIS targets in Syria, in response to the US downing a Syrian fighter jet near Raqqa.
In Syria, an increasingly complex battlefield situation has sometimes set the US and Russian at odds. Russia has offered few details about its operations, and the US-led coalition has had to keep a closer eye on Russian subs in the eastern Mediterranean.
The Krasnodar didn't threaten the Bush during these operations. But subs are generally hard to detect, and one like the Krasnodar attracts special attention. Its noise-reducing abilities have earned it the nickname "The Black Hole."
"One small submarine has the ability to threaten a large capital asset like an aircraft carrier," US Navy Capt. Bill Ellis, commander of US anti-sub planes in Europe, told The Journal.
Russia has beefed up its naval forces considerably since 2000, seeking to reverse the decline of the 1990s.
The Krasnodar marked an advancement in Russian submarines, and more a new class of subs — designed to sink aircraft carriers — is now being built, according to The Journal.
While Russia has gotten better at disguising its subs, the US and Western countries have kept pace with enhanced tracking abilities.
An SH-60F Sea Hawk helicopter assigned to Helicopter Anti-Submarine Squadron 15 flies past the Nimitz-class aircraft carrier USS Carl Vinson during an air power demonstration. (U.S. Navy photo by Mass Communication Specialist 2nd Class James R. Evans)
"We are much better at it than we were 20 years ago," Cmdr. Edward Fossati, who oversees the Bush's anti-sub helicopters, told The Journal.
But the Krasnodar's Mediterranean maneuvers appeared to meet Moscow's goals, striking in Syria while avoiding Western warships.
Moscow's naval activity around Europe now exceeds what was seen during the Cold War, a NATO official said this spring, and NATO and Russian ships sometimes operate in close quarters around the continent.
When the UK sent its new aircraft carrier, HMS Queen Elizabeth, to sea trials in June, navy officials said they expected Russian submarines to spy on it. During US-UK naval exercises — in which the Bush participated — off the coast of Scotland in August, a Russian submarine was spotted shadowing the drills.