The wall at that spot is older than almost everything it has ever kept out. Aurelian's builders raised it in the 270s: a core of concrete with a facing of thin red brick, courses laid flat and level, the mortar beds nearly as thick as the bricks themselves. Sixteen centuries of patching followed. Popes refaced it, heightened it, cut it, blocked windows and opened new ones, and each repair used whatever brick was to hand, so the colour shifts along the curtain like weather.
On this day in 1870 Italian artillery worked on a stretch just north of the Porta Pia for a few hours of the morning and brought down roughly thirty metres of it. The Bersaglieri went in through the gap. A few dozen men died on both sides before the white flag went up, which is few for a capital and not few at all if you were one of them. The plebiscite came on 2 October; Rome was made the capital the following February.
What I like is what happened after. Masons came with scaffolding and rebuilt the breach, and they did it honestly rather than invisibly — the new brick sits in its own tone, the coursing meets the old at a seam you can follow with a finger if you stand back on the pavement and let your eye run along instead of at. In the photograph below, the travertine gateway with its Latin inscription and the mosaic Madonna above is the face the city gave itself; the patched wall is the part nobody signed.
Fabien, the seam had a counterpart in the men who stayed inside it: Pius IX withdrew into the Vatican and declared himself a prisoner there, and his successors kept to that for nearly sixty years. Catholics were long discouraged from voting in the new kingdom's elections, and the question was only settled by treaty in 1929.
Fabien, the plebiscite on 2 October settled it in the ledger as the artillery had settled it on the ground, and Rome was declared the capital the following February. Governments move in seams too, and they rarely match the old coursing either.