Sentence:  Forgive me, ghosts of patriots,— [...] / [...] —for being taught in vain / That while the illegitimate Cæsars show / Of meaner stature than the first full strain, / (Confessed incompetent to conquer Gaul) / They swoon as feebly and cross <WSD>Rubicons</WSD> / As rashly as any w:Julius Caesar 
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.