Thank you for remembering, lexico...
Though when I saw the title, I thought you might have known that the 5th of October is not only the anniversary of the Republic in October 5th, 1910, but also of the Monarchy, because the Kingdom of Portugal was (at least officially, though historians now think otherwise) proclaimed on October 5th, 1139.
This odd coincidence comes from the fact that, in 1910, the republicans had decided to stage their anti-monarchist coup during the celebrations of the day of the Kingdom, thus surprising the Kings' forces.
It turned out to be a bloody stupid idea, mostly because the Royal Army was out in force in the streets that day, in order to take part in the ceremonial parades.
So, the republican forces soon found themselves outnumbered 4 to 1 and quickly surrounded in a famous location in the city, and by midday their position seemed hopeless. Losing all hope, one of the main republican leaders (Admiral Cândido dos Reis) committed suicide.
Then something happened that nobody expected.
The German ambassador, afraid of the potential danger posed by the combats to the german citizens living in Lisbon, asked the Royal commander for a small truce, during which the germans could be evacuated. The general agreed.
But the republicans also had to be informed. So, the ambassador ordered a rider to go and meet them in order to arrange the truce. To show his intentions, the rider should carry a large white flag.
To reach the republican positions, however, the rider had to go from the royal HQ to their location, which implied moving up all the way through one of the main - and longest - streets.
When everyone saw a large white flag moving from the royal side, the citizens - who were 80% pro-monarchy - started to say: "the Army is surrendering!". And these cries reached the royal soldiers, who thought "looks like we surrendered".
By the time the rider reached the republican positions, their soldiers were saying "they've surrendered! We won!", and without stopping for consultation, their commander led an advance directly to the royal HQ. They were not stopped by the royal troops, who were putting down their weapons, convinced that a surrender had been ordered.
It was quite a surprise for the royal general to see the republicans burst into his HQ less than an hour after he had gently offered a truce. He still tried to protest that it *wasn't* a surrender but a truce, but it was too late - he was captured.
Seeing the debacle of his troops, the King hurriedly left the Palace and the capital, and later that day the Republic was proclaimed.
And we owe it all to Germany :relief: