The planes had been following this route for years, but by the 1980s, radar systems had gotten much more powerful.
"Devising a new plan for intercepting the SR-71s, a frankly terrifying, and un-orthodox strategy was devised," the HotCars report said. "Scrambling from their bases, Viggen fighters would enter a steep climb to reach an altitude just below the SR-71, accelerate to twice the speed of sound, then climb again to fly directly towards the SR-71 in a head-on approach, almost like a game of chicken. While typically intercepts would approach from behind, allowing missiles a better chance to lock on, the Viggen's Skyflash missile was capable [of] using its radar to lock on from the front, making the head-on attack the Viggen's only real window for an effective missile lock, as intercepting the Blackbird from behind was an impossible task."
Posted April 1, but not a joke. The photo is a joke, however, since it's a Russian SU-34, from the archive. I could not find a Viggen or a Blackbird.
This led to January 1986, when a Swedish pilot named Per-Olof Eldh, using a Viggen fighter jet, lined up with the Blackbird's flight path and even gained full missile lock. However, Eldh did not fire, and the two planes ended up merely crossing paths. The same pilot would go on to have five more interceptions using the same strategy. "The point had been proven [and] it was clear the Blackbird could no longer fly wherever it pleased without being challenged."
However...
Then, in 1987, an SR-71 veered off-course into Swedish airspace and descended dramatically. Flying alongside it, the Swedish planes realized one of the jet engines on the Blackbird had exploded, so the Viggens protected the Blackbird, escorting it out of Swedish airspace. That mission would be declassified many years later, in 2017, leading to the U.S. Air Medal for Bravery being awarded to the Swedish pilots in 2018.