Using a manhole cover to disable a tank works well if you can get it into the treads, but Molotov cocktails are more effective against trucks – such are just a couple of the discoveries unearthed by Czech filmmaker Jan Sikl in his docu “Reconstruction of Occupation,” a granular look at the 1968 Warsaw Pact invasion of Czechoslovakia based on never-before-seen footage. The documentary premiered at the Karlovy Vary Film Festival on Saturday.

The Soviet-led crackdown, ordered in response to the reforms of hardline communism ushered in by Alexander Dubcek during the Prague Spring of that year, marked a historic turning point that brought citizens of Czechoslovakia who had hoped for a lessening of brutal repression back into line for another 20 years. It’s been thoroughly studied and was remarkably well documented by filmmakers at the time, many having escaped to the West with film reels hidden in wheel wells of cars, much as things transpired in movies such as “The Unbearable Lightness of Being.”

But what Sikl found over three years of gathering forgotten material – from both official footage locked up in vaults and amateur 8mm stock – illustrates scores of small, heroic moments as ordinary people stood up to tanks and troops while others made careful calculations about the risk of confronting the invading armies.

“I didn’t want to make a moral judgment,” Sikl says, explaining that it was important to him to interview both those who collaborated and those who resisted.

While he admits none of the footage caused him to reconsider the fundamental facts and events of Aug. 20-21, 1968, he says the material he found has other qualities that bring to life the appalling realities of witnessing a peaceful European capital overrun without warning by presumptive allies.

The official Moscow line, that the invasion was an emergency response to some manner of urgent threat from Western imperialists, convinced no one at the time but the hand drawn signs waved in Prague and other nearby cities expressed the clear sense of betrayal: “Dear to us yesterday – murderers today,” reads one.

While the Soviets certainly signaled that leadership was not happy with Czechoslovak First Secretary of the Communist Party Dubcek’s “Socialism with a Human Face” policies – including new press and political freedoms – army and air bases in Bohemia were caught completely off-guard by the massing of troops from the East bloc on local roads. Footage in Sikl’s docu shows MiG fighters waiting on the ground as fretful officers stand on high alert, awaiting instructions.

“There was terrible confusion among the Czechoslovak military,” Sikl says – and it wasn’t limited to the victims of the occupation. Another discovery was perplexing footage of a Soviet tank hopelessly entangled in the wreckage of a small bridge, half immersed in stream water.

The scene does not depict a forgotten battle – rather, the tank crew apparently misunderstood the Czech warning sign on the bridge reading “5T”, signifying the five-ton weight limit. “They must have thought it meant five tanks,” Sikl’s ironic voiceover intones in the film.

Images of young people in Prague gathered around tanks – often those operated by conscripts from the East who only vaguely understood where they were or what their mission was – is a common sight in historic accounts of the events of that summer. But few up to now have seen the more perplexing images of surprised army officers in Hradec Kralove trying to work out how to minimize bloodshed while retaining a shred of their dignity.

Sikl’s compilation, accompanied by eye-witness oral accounts and an eerie soundtrack composed by Sikl’s son Jan Sikl Jr., also shows an inspiring account of protesters in Pilsen fighting to keep the radio station there from being shut down as Czech Radio was in Prague. In another segment, a woman who witnessed a young man shot down for waving a Czechoslovak flag describes picking it up and carrying on marching down the street, hoisting the now blood-stained tricolor standard.

Indeed it was the eyewitnesses and those who recognized themselves or their family members from the footage Sikl believes are the greatest treasure trove, he says. The biggest bulk of footage, 35 boxes of high-quality 35mm film, were found in the garage of a man who says it was left there by a mysterious friend. Beyond that, its origins are never clearly established.

That main find was definitely shot by professionals, likely military or security camera crews, Sikl says, though none has until now been released to the public. After making the discovery, Sikl appeared on Czech TV, appealing to viewers to help identify people and events in the footage.

Over 1,000 responded, many handing over new amateur footage from their own attics and storerooms.

The authenticity of the 35mm reels – and the reason so much of it was locked away for over five decades – is clear the moment it’s threaded into a projector. Some reels show the aftermath of the Warsaw Pact invasion as Czechoslovak security forces are trained in suppressing protests with water canons and weapons – techniques they learned well enough to deploy decades later against protesters during the 1989 Velvet Revolution.