The most famous boundary in Roman history was a small stream named for its color. Rubicon comes from the Latin Rubico, built on a word for red.
Sources disagree slightly about the red. Wikipedia derives the name from rubeus, "red," and says iron deposits in the riverbed tint the water. Etymonline gives rubicundus, "ruddy," and credits the reddish soil on its banks. Both agree that the river was named for its color.
In Caesar's day the Rubicon ran to the Adriatic and marked part of the southern edge of Cisalpine Gaul, the province he governed. Beyond it lay Italy proper. Bringing an army across it broke the law that limited a governor's military authority, his imperium.
Around 10 January 49 BC, Caesar led the Thirteenth Legion over the river anyway. He was moving against Pompey, and the crossing made armed conflict unavoidable. The historian Suetonius has him say alea iacta est, "the die is cast." Plutarch instead reports a Greek line from the playwright Menander, "let the die be cast." Either version says the same thing: the throw is made and cannot be taken back.
English began using the river as a metaphor in the 1620s, according to Etymonline. To cross or pass the Rubicon came to mean taking any step that commits you for good. It needs no army, just a point of no return.
The river itself became a puzzle. Several small local streams were candidates, and Renaissance scholars argued over them. A forged inscription even persuaded some that the Pisciatello was the real one. In 1933 Italy officially identified the Fiumicino, near Savignano di Romagna, as the ancient Rubicon. In 1991 three Italian scholars added strong evidence for it, using distances on the Tabula Peutingeriana, an ancient map.
The color has relatives all over English. The same Latin family of red words gives us rubric, from red ochre, because headings in old books were written in red ink. Ruby and rubicund come from the same red root.