Wasp-class amphibious assault ship USS Kearsarge of the U.S. Navy is seen at right, next to the HSwMS Belos submarine rescue ship of the Swedish Navy (left) in the harbor in Stockholm, Sweden, in a June 3, 2022, file photo taken ahead of a NATO exercise in the southern Baltic Sea. HENRIK MONTGOMERY/TT News Agency/AFP/Getty

A rising number of mysterious drone sightings near airports and military bases has fueled concern in Europe over alleged Russian incursions into NATO airspace, with some of America's allies already saying the continent is in a gray zone between peace and war — accusing Moscow of escalating "hybrid warfare."

NATO, the North Atlantic Treaty Organization, is arguably the United States' most important alliance, enduring for over 75 years. In Sweden, CBS News saw that alliance in action this week — in waters contested by Russia.

Our team watched as an enemy submarine prepared for a covert espionage and sabotage mission in the Baltic Sea against America's NATO allies in northern Europe.

It was just a military exercise, launched from Stockholm's harbor, and a German submarine played the role of the unnamed enemy being hunted by NATO forces across the Baltic Sea.

U.S. forces took part, flying overhead in spy planes.

"NATO is a defensive alliance," Commander Arlo Abrahamson, a U.S. Navy officer and a spokesman for NATO's maritime headquarters, which has been closely monitoring Russia's military build-up in the Baltic, told CBS News. "The potential threats of adversaries in this region are interconnected throughout the world."

Abrahamson said that if northern Europe is drawn into a conflict, it would have a negative impact on the United States, too.

Several of America's NATO allies share a Baltic coastline with Russia. In recent years, Russia is suspected of carrying out multiple acts of sabotage — including damaging undersea data cables that are the backbone of the global internet.

Some believe Russia is using the Baltic Sea as a testing ground, to see how much damage it could do to the West's economy if it ever went to war with NATO. Undersea cables carry trillions of dollars worth of financial transactions every day.

The country leading the exercises this week, Sweden, gave CBS News rare access to one of its stealth warships, the HMS Helsingborg, as it hunted for the pseudo-enemy sub.

For more than 200 years, Sweden maintained a policy of military non-alignment, but last year, following Russia's 2022 invasion of Ukraine, it became NATO's newest member.

"We face the Russians every day, sharing the same duck pond, so to say," submarine flotilla Commander Paula Wallenburg, of the Swedish navy, told CBS News.

She echoed Lithuanian officials' recent accusation that Russia is engaged in a kind of hybrid warfare with the alleged airspace violations and acts of sabotage.

"We're not at peace, but not at war," she said. "We're somewhere in between."

Wallenburg agreed that the current circumstances look "pretty close to" those seen during the Cold War, when the nuclear-armed U.S. and then-Soviet Union tested each other's resolve in a high-stakes face off that never quite turned into a full-scale war.

"It's a very serious situation when it comes to security here in this area," she said.

The Kremlin has actually said already that Russia is at war with NATO, because of the alliance's support for Ukraine.

America's NATO allies around the Baltic Sea, including Sweden, Finland, Poland and Germany, have all resolved to significantly boost their domestic military spending and beef up their defenses.

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