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The Rubicon isn't much of a river, but it meant a great deal to Julius Caesar.
After all, the Roman Senate had forbidden any of its generals in foreign provinces from crossing those waters and bringing their forces into Rome. To do so would be to commit treason.
But General Caesar was already at odds with the Senate. In 49 B.C., he crossed the Rubicon and went on to rule the Empire. These days, "crossing the Rubicon" means passing a point of no return.
For example, as one Twitter user remarked, "Novel [online] social applications seem useless until they cross the Rubicon and begin to be indispensable." (davidweinfeld, April 8, 2010)