Sentence:  Fortunately for England ſhe is yet on the peaceable ſide of the <WSD>Rubicon</WSD>; but as the flames once kindled are not alway eaſily extinguiſhed, the hopes of peace are not ſo clear as before the late myſterious diſpute began. 
Meaning of the word marked with <WSD> tags:  A limit that when surpassed cannot be returned from, or an action that when taken cannot be reversed.