I'd say East Germany, November 1989.

There had been protests for a month in the streets starting in Leipzig and spreading throughout the GDR, and the fact that thousands of citizens were in the streets protesting seemed to have worried the government enough to loosen the reins somewhat. Still, their goal was not to bring down the Wall; they were trying to maintain the East German regime while offering a minimum of reform.

I don't think anyone knows how it would have ended if it hadn't been for a massive error in communication that, when combined with the power of a mass of people, essentially brought down the Wall with no violence or weapons whatsoever.

On Nov. 9, 1989 Günter Schabowski, one of the members of the Politburo, said on the radio that East Germans would be allowed access to West Germany now. "Effective immediately." This was a miscommunication and certainly not what the leadership had decided on; the Politburo intended to open up the visa process and make travel to the West easier.

But many Berliners who heard Schabowski's press conference decided to take him at his word. They started gathering at the Bornholmer Strasse border crossing in Berlin, and the guards there were quickly very much outnumbered. The guards had heard the broadcast as well, and as more and more people gathered, the guards obeyed their initial orders and warded off the crowd, and then attempted to allow some organized border crossing (stamping passports and all the usual bureaucracy included). But there were just too many people there. The senior guard at the post recognized that the situation was extremely risky, and that there was a high potential for violence. He made the decision - against the orders from higher-ups - to open the gates and let people through.

This isn't my academic area - what I know comes primarily from avid newswatching at the time and a lot of personal histories from friends who lived on both sides of the Wall - and having lived through it, it's hard to think of it as strictly "history" instead of "news," as the sources below betray. This also means this answer may not be up to the standards of the sub and need to be deleted; mods, I apologize if this is the case.

"Bornholmer Strasse: Der Mauerfall: Eine Rekonstruktion." Mitteldeutscher Rundfunk. Originally published 2014, updated 9. Nov. 2022.

Michael Meyer, "Günter Schabowski, the man who opened the Berlin Wall." New York Times 6. Nov. 2015.

"Fall of the Berlin Wall: How 1989 Reshaped the Modern World." BBC News, 5. Nov. 2019.

The birth of the Second Spanish Republic, 14th of April 1931. It was successful in changing the regime. ^(It wasn't so successful after that.)

Spain had been under a constitutional monarchy from 1876 to 1923, a highly unstable period, economically, politically and socially. In 1923, the system finally collapsed, and a military coup took place, the king rapidly handing power over to the military. In 1929, Miguel Primo de Rivera (the dictator who had governed since the coup) resigned and the king appointed another head of government to try to restore the "constitutional" monarchy.

However, king Alfonso XIII was very unpopular and seen as an accomplice of the dictatorship and partially responsible from the 1909-1923 political instability. Returning to the previous situation as if nothing had happened was not a real option. The political opposition (republicans, Catalan nationalists, socialists, and even some center-right Christian liberals) formed a revolutionary committee to plan the overthrowing of the monarchy.

In the 12th of April 1931, municipal election were held (to renew mayors of all Spanish cities) and the republican and socialist parties won in 41 of 51 provincial capitals.

The 14th, when the election results were known, huge (unarmed) crowds took to the streets waving the Republican tricolor and calling for the establishment of a Republic. The Republic was proclaimed in many Spanish cities (by the councilors elected in the municipal election). In Madrid, the capital, demonstrators concentrated in front of the Royal Palace, ministry of Governación ("Homeland Security") and the Post Office.

The Revolutionary Committee requested to be handed power and for the king to leave the country. The head of the Civil Guard (Spain's military police body) told the government that it wasn't possible to maintain public order in Madrid, and that if he ordered his men to shoot on the crowds, they wouldn't do so.

The Monarchist Government had no other option than to hand out power. The king left the country that same day, and before night, the new republican Government was already holding its first Council of Ministers and writing its first decrees.

So, in less than 24 hours Spain had gone from a monarchy to a republic. All of that without a shot fired, thanks to the municipal elections (whose goal was to calm down the situation) and the popular demonstrations, which left the government unable to respond.