The fog of propaganda and counter-propaganda settled long before the last corpse had been laid to rest. One side called the catastrophe a deliberate act of warfare, an unspeakable crime that stood as testimony to the enemy’s disregard for human life. They called for immediate retaliation from the international community.

Not at all, said the other side: all this disaster proved was western civilisation’s hypocrisy and decadence. It finally exposed the illusion that wars could be led at arm’s length. There were already rumours that it had all been a set-up, a “false flag” designed to pull American muscle into what had until then looked like a European conflict.

These are the competing summaries of what happened after Malaysia Airlines flight MH17 was shot down over Ukraine last year, killing all 298 passengers and crew. But they could just as well refer to another catastrophe that took place 100 years ago this week.

On 7 May 1915 the British ocean liner RMS Lusitania sank to the bottom of the Irish sea after being hit by a German U-boat’s torpedo, killing 1,198 civilians – an event that not only helped shift American attitudes to the war, but also altered the rules of warfare.

Last summer’s MH17 attack ratcheted up the pressure on Vladimir Putin’s government, but curiously it has faded from public view since. This is partly because it was bracketed by two other aviation disasters – the mysterious disappearance of Malaysia Airlines flight MH370 and the Germanwings crash in March – and partly because it attracted so many conspiracy theories so quickly that the idea of establishing any kind of objective truth about the event soon seemed hopelessly ambitious.

Within hours, there were rumours that the plane had been downed by a US rocket, in a botched attempt to disable Vladimir Putin’s private jet, or that the plane had in fact been pre-filled with corpses. Theories were instantly met with counter-theories. In some cases, the same footage (with minor Photoshop amendments, as the blogger Eliot Higgins has shown) was used by both Ukrainian and Russian authorities to blame the other side. Some, like the writer Peter Pomerantsev, have suggested that this is part of the Kremlin’s new, postmodern propaganda strategy to destabilise the very notion of objectivity itself, “a kind of linguistic sabotage of the infrastructure of reason”.

But the Lusitania shows that this instant blurring of fact and fiction is nothing new. In Hamburg’s maritime history museum, there is a medal made by the German expressionist artist Karl Goetz in August 1915, which depicts Death selling tickets for the Lusitania to ordinary workers, under the slogan Geschäft über alles – “Business above all”.

The story of the Lusitania reminds us that conspiracy theories grow more powerful with time

German propaganda alleged within days of the sinking that the Brits had been perfectly happy to risk lives by sending a passenger ship carrying war munitions through a war zone, as long as they could make a healthy profit. The fact that the Lusitania sank so quickly – in 18 minutes, as opposed to the Titanic’s three hours – is usually attributed to a mysterious “second explosion”, which the German government at the time took as proof the ship was carrying ammunition.

The same medal can also be found in the British Museum. Why? Because like Russian media makers a century later, the Brits realised that the most effective counter-propaganda repossessed the enemy’s own images, and promptly commissioned the production a quarter of a million copies. All they needed to do was to add a little note that said: “This indicates the true feeling the War Lords endeavour to stimulate, and is proof positive that such crimes are not merely regarded favourably, but are given every encouragement in the land of Kultur.”

On the one hand, the story of the Lusitania reminds us that conspiracy theories grow more powerful with time. Gregg Bemis, an American multimillionaire who bought the Lusitania’s wreck in 1982, seems intent on showing that the ship carried more ammunition than previously admitted, regularly organising dives to the wreck to prove his point. The US author Erik Larson’s new book on the sinking, Dead Wake, gives some considerable credence to the theory that Winston Churchill, if not actively trying to force the catastrophe in order to bring America into the war, seemed at least to have accepted the risk as a strategic convenience.

On the other hand, the Lusitania also teaches us that there are stories and theories, and then there are certain moral truths. It is true that chance may have played as much of a part as deliberation in both these catastrophes. According to US intelligence reports, flight MH17 was hit by a ground-to-air missile fired by pro-Russian rebels trying to down Ukrainian fighter jets, which they had managed to do in the same area three times earlier that same month. The Buk missile system that they most likely used has a radar which does not allow those manning it to distinguish civilian planes from fighter jets.

Even if pro-Russian rebels made a mistake, someone enabled their mistake to have such monstrous consequences

The Lusitania, similarly, was torpedoed by an old-fashioned, pre-radar submarine – a “one-eyed vessel”, as one German U-boat commander described it, in which the decision whether to fire or not was based on one pair of and their distant view through a foggy telescope. According to the submarine’s logbook – which may have been tampered with in the diplomatic aftermath – the crew only realised they had attacked a civilian vessel after their missile had struck, with the pilot crying out: “My God, it’s the Lusitania.” So in 1915, just as in 2014, there was a temptation to write off the lost lives as a stroke of monumental bad luck rather than a war crime.

But the Lusitania also shows that confusion is usually borne out of calculated risk. In 1915 Germany’s supreme military leader, Kaiser Wilhelm, had authorised U-boat commanders to sink any ship they had reason to believe to be British or French, and to do so without warning, while still submerged. The accident, in other words, was waiting to happen. Pro-Russian rebels in the Ukraine, likewise, must have acted under orders; weapons experts agree that they would have lacked the know-how to launch the missile. Even if they made a mistake, someone enabled their mistake to have such monstrous consequences.

Conspiracy theories about the “second explosion” will run and run, but when it comes to the right or wrong of what happened, they are mostly irrelevant. In Wilful Murder, her meticulously researched history of the Lusitania, the historian Diana Preston points out that while the ship did indeed carry ammunition and other contraband, under “cruiser rules” this would have justified seizing the cargo and destroying the vessel only after guaranteeing the safety of its passengers. Sinking the Lusitania on sight, however, took the barbarity of war to a new level.

A Dutch investigation into the MH17 crash will present its findings in October. It deserves the world’s full and uncynical attention.