Rubicon

The Rubicon was a river between Italy and the province of Cisalpine Gaul. It was named for its red colour, which was caused by its bed of iron-rich clay. (PROSE: The Ruby's Curse)

In 49 BC Julius Caesar refused to disband the army and crossed with it into Italy, an act which broke Roman law and incited a civil war. (PROSE: The Ruby's Curse) The crossing of the Rubicon was considered a fixed point in time. attempted to cross in Caesar's place only for the Seventh Doctor to thwart his plan. (COMIC: Crossing the Rubicon) River Song was also present at the crossing. (PROSE: The Ruby's Curse)

The Tenth Doctor said to Rose Tyler that he could take her to see "Caesar crossing the Rubicon". (TV: Tooth and Claw)