RUBICON  , a small stream of

ancient Italy, which flowed into the Adriatic between Ariminum and Caesena, and formed the boundary between Italy and the province of Cisalpine Gaul . Hence Caesar's crossing of it in 49 B.C. was tantamount to a declaration of war against Rome as represented by Pompey and the Senate . The historic importance of this event gave rise to the phrase " crossing the Rubicon " for a step which definitely commits a person to a given course of action . There has been much controversy as to the identification of the stream; it appears that its upper course is represented by that of the Pisciatello (called Rubigone in the 11th or lath century and now Rugone or Urgone), and its lower portion by the Fiumicino, which the Urgone once joined . The point was marked by a station on the Via Aemilia below their confluence, 12 M . N.W. of Ariminum, bearing the name ad Confluentes; and here is still preserved a three-arched bridge, larger than is necessary for the water carried by the present Fiumicino .