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... testis pulcherrima animum exstimulaverunt, nisi impunitatis cupido retinuisset, magnis...

Another of Tacitus's generalizations put into a past contra-factual, conditional sentence with the apodosis replaced with a fact: (maybe?) the conspiracy would have succeeded, "if it had not been eager for impunity, (which is) always adverse to those engaged in great struggles."