The tweet was sent by Germany’s federal agency for migration and refugees a year ago today. “The #Dublin procedure for Syrian citizens is at this point in time effectively no longer being adhered to,” the message read. With 175 retweets and 165 likes, it doesn’t look like classic viral content. But in Germany it is being spoken of as the first post on social media to change the course of European history.

Referring to an EU law determined at a convention in Dublin in 1990, the tweet was widely interpreted as a de facto suspension of the rule that the country in Europe where a refugee first arrives is responsible for handling his or her asylum application.

#Dublin-Verfahren syrischer Staatsangehöriger werden zum gegenwärtigen Zeitpunkt von uns weitestgehend faktisch nicht weiter verfolgt. — BAMF (@BAMF_Dialog) August 25, 2015

By this point in 2015, more than 300,000 asylum seekers had reached Europe by boat – a figure that was already 50% higher than even the record-breaking number of arrivals in 2014.

Although the German agency’s intervention certainly did not start the crisis, it did make Germany the first-choice destination for Syrians who previously might have aimed for other countries in Europe, such as Sweden, which at the time offered indefinite asylum to Syrians.

It also created an impression of confusion and loss of political control, from which Angela Merkel’s government has at times struggled to recover. Twelve months on, politicians and officials at the centre of Berlin’s bureaucratic machine are still trying to figure out how the tweet came about.

Four days previously, Angelika Wenzl, the executive senior government official at the refugee agency, which in Germany is known as BAMF, had emailed out an internal memo titled “Rules for the suspension of the Dublin convention for Syrian citizens” to its 36 field bureaux around the country, stating that Syrians who applied for asylum in Germany would no longer be sent back to the country where they had first stepped on European soil.

Lawyers working closely with BAMF have pointed out that doing so did not, as some have claimed, amount to a complete suspension of the Dublin agreement across the EU, since the convention gives member states the right to take over asylum applications from other member states.

By channels that officials and journalists have so far failed to pinpoint, Wenzl’s internal memo was leaked to the press. While an investigation by Der Spiegel pointed at Germany’s largest immigration advocacy group, Pro Asyl, as the source of the leak, the NGO itself claimed it did not know of the memo until told about it by journalists seeking to make sense of the new procedure.

Maximilian Pichl, Pro Asyl’s legal policy adviser, said he was one of many lawyers who flooded BAMF with puzzled phone calls, creating pressure on the agency to clarify its position in public, which eventually culminated in what Die Zeit recently described as “the fateful tweet”.

More by old-fashioned word of mouth than by means of the retweet button, the message spread among refugees who were heading to Europe or already waiting in camps. “Right now it’s just one country – Germany,” a Syrian oil engineer told the Guardian as he made his way through the Balkans a few days later. “Where are the others? It’s only Germany. Only Merkel.”

Refugees at Vienna’s railway station in September 2015. Photograph: Joe Klamar/AFP/Getty Images

Chinese whispers spread through the Syrian diaspora, inflating the significance and meaning of the government’s tweet. “She said she will bring big boats from Turkey to rescue Syrians!” Maria, a Syrian interviewed in Vienna station that fortnight, credulously explained.

Until mid-August 2015, 150,000 refugees had been registered in Hungary. After BAMF’s tweet, many refused to do so, reportedly holding up their smartphones displaying the message to police and border officers. Hungary’s ambassador to Germany later claimed that the day after the tweet, Serbian police had found thousands of discarded passports on their side of the border. “From that moment on, every refugee was a Syrian,” Peter Györkös said.

When Györkös called the German interior ministry, officials said they were not aware of the tweet. In a press conference the following day, Germany’s interior minister, Thomas de Maizière, said that the suspension of the Dublin agreement was “not as such a legally binding act”, but more of a “guideline for management practice”.

A few weeks later, Manfred Schmidt, the head of BAMF, stepped down for “personal reasons”, although in many quarters it is assumed that he was forced to do so because of his agency exposing the government’s loss of political control.

But a year on, the long-term consequences of the tweet are gradually becoming apparent. In April, the European commission revealed a wide-ranging shake-up of the Dublin system, which has long been criticised by human rights lawyers for unfairly pushing the main burden of asylum applications to poorer countries on the periphery of the EU while protecting wealthier landlocked member states.

Yet sources close to BAMF insist the tweet had not been intended to torpedo the unloved law, but as an emergency decluttering measure, freeing the agency from additional bureaucracy before it was unable to cope.

According to Gerald Knaus, the initial architect of the EU-Turkey deal and the head of the European Stability Initiative thinktank, the tweet was not intended to signal a major policy change, and was not written by senior policymakers.

“It was seen as simply stating the obvious – they had already stopped trying to send Syrians back – and it wasn’t considered how it would be perceived,” said Knaus. “It took on a life on its own, because it confirmed that anyone who got to Germany could stay. It wasn’t a new decision – but it sent out a signal.”

Behind the scenes at the time, Knaus said, German officials were split between those who felt it was necessary to send people back to Hungary and viable to close the German border, and those who felt that it was logistically impossible and morally unjustifiable. A few weeks later, when thousands of Syrians decided to march from Hungary towards Austria and then Germany, Merkel decided to opt for the latter.