More on Bornholm's specifics in a moment, but first let's cover one more example of Putin Russia's aggressive wrongdoing. According to an open-source Danish security assessment, in mid-June 2014, three months after Putin's Kremlin attacked and annexed Ukraine's Crimean peninsula, Russian aircraft carrying live missiles bluffed an attack on Bornholm. Though the report doesn't provide the exact date, the bomber "probe" occurred during the three-day period the island hosted a touchy-feely "peoples festival." The festival's 90,000 participants were unaware they were seeking peaceful solutions on a bulls-eye.
The Bornholm faux-attack reprised Soviet Cold War "tests" of Danish defenses and is but one of a score of serious Russian military probes since 2008 designed to rattle Northern Europe. These Kremlin air and naval probes, backed by harsh rhetoric, have led Denmark, Norway, Sweden and Finland to reassess their military defenses. "Nordic cooperation" with an emphasis on territorial defense was the first formulation. The Nordics, however, acknowledged ties to Baltic states (and NATO members) Estonia, Latvia and Lithuania. Since the Crimea invasion, Denmark and Norway (NATO members) want to reinvigorate NATO military capabilities. Continued Russian aggression in Ukraine has led a few habitually neutral Swedes to voice an interest in joining NATO.
Yes, Putin-era Kremlin belligerence has sparked a sea change in Sweden. When regional powers make political demands, smaller nations always react. Often they accommodate, perhaps appease. However, when a regional power actually pulls its sabers -- and especially when the bully makes territorial threats -- good golly, sometimes the small nations don't cower. This is a global, trans-cultural phenomenon major media tend to miss. Aggressive Chinese behavior in the South China Sea has spurred Southeast Asian nations (especially Vietnam) to consider mutual defense cooperation. Here's another wrinkle. Given Iran's nuclear weapons quest and its violent trouble-making throughout Southwest Asia, Saudi Arabia and Egypt are discussing deploying a pan-Arab army.
Back to Bornholm: the island's location and geology irritated Soviet-era Kremlin strategists. Located in the Baltic Sea east of peninsular Denmark, north of Poland's coast and to the rear of what was East Germany, Bornholm gave the Free World outpost north of and behind Warsaw Pact lines.
Soviet communications security officers despised the place. Bornholm's electronic intercept systems, quite literally, bugged them.
As for geology, unlike Jutland's flat peninsula, Bornholm is rock. In the 1970s, while serving a U.S. Army tour in West Germany, I heard a senior officer describe Bornholm as "sort of a Gibraltar." His exaggeration had a point. Dig tunnels and Bornholm became a hard target for Soviet conventional weapons.
From Bornholm, Danes monitored the Polish coast and Soviet warships in the Baltic. The Soviet Navy facility at Kaliningrad was of particular interest.
Bornholm is still north of Poland, but the political geography has changed. Poland is now a NATO member, and one demanding an alliance military buildup. Poland and Lithuania surround Kaliningrad.
Why bluff bombing Bornholm? Why test Norwegian and Swedish defenses?
Denmark strongly supports Poland. Poland supports Ukraine. Thus Putin's Kremlin marks Danes as enemies. Weakening Danish resistance to Russian adventurism politically splits NATO and may delay NATO military modernization.
The Kremlin knows the small nation that resists can become a huge problem. Putin hero Josef Stalin learned that in 1940 (as an ally of Nazi Germany) when he ordered the Red Army to attack Finland. In The Winter War, Finn ski troops cut Russian regiments to shreds. Finns of 2015 believe Danes, Swedes, Norwegians and maybe even Americans have their back. Putin, however, is dead set on seeding doubt.