Modern historians are divided on the question whether or not Crassus and Caesar were behind Catilina – and how serious the conspiracy was, for that matter.
Theodor Mommsen in History of Rome has no doubts about either (p. 62 ff), at least regarding the events of 63. He sits on the fence about the earlier "first" conspiracy:
Certainty cannot be attained on the point; but there is a great probability that Crassus and Caesar had projected a plan to possess themselves of the military dictatorship during the absence of Pompeius; that Egypt was selected as the basis of this democratic military power; and that, in fine, the insurrectionary attempt of 689 had been contrived to realize these projects, and Catilina and Piso had thus been tools in the hands of Crassus and Caesar.
Mommsen's opinion about Crassus and Caesar is clear in this section:
Attitude Of Crassus And Caesar Toward The Anarchists (pp. 69 & 70)
[...] All these pieces of evidence speak clearly enough; but, even were it not so, the desperate position of the democracy in presence of the military power--which since the Gabinio-Manilian laws assumed by its side an attitude more threatening than ever--renders it almost a certainty that, as usually happens in such cases, it sought a last resource in secret plots and in alliance with anarchy. The circumstances were very similar to those of the Cinnan times. While in the east Pompeius occupied a position nearly such as Sulla then did, Crassus and Caesar sought to raise over against him a power in Italy like that which Marius and Cinna had possessed, with the view of employing it if possible better than they had done. The way to this result lay once more through terrorism and anarchy, and to pave that way Catilina was certainly the fitting man. Naturally the more reputable leaders of the democracy kept themselves as far as possible in the background, and left to their unclean associates the execution of the unclean work, the political results of which they hoped afterwards to appropriate. Still more naturally, when the enterprise had failed, the partners of higher position applied every effort to conceal their participation in it. And at a later period, when the former conspirator had himself become the target of political plots, the veil was for that very reason drawn only the more closely over those darker years in the life of the great man, and even special apologies for him were written with that very object.
As to Cicero, he has this to say (on the execution of the Catlinarians):
But it was a dreadful deed, and all the more dreadful that it appeared to a whole people great and praiseworthy.
Never perhaps has a commonwealth more lamentably declared itself bankrupt, than did Rome through this resolution--adopted in cold blood by the majority of the government and approved by public opinion--to put to death in all haste a few political prisoners, who were no doubt culpable according to the laws, but had not forfeited life; because, forsooth, the security of the prisons was not to be trusted, and there was no sufficient police. It was the humorous trait seldom wanting to a historical tragedy, that this act of the most brutal tyranny had to be carried out by the most unstable and timid of all Roman statesmen, and that the "first democratic consul" was selected to destroy the palladium of the ancient freedom of the Roman commonwealth, the right of provocatio.
When reading Mommsen, one has to keep in mind his own political involvement in the failed democracy movement in 19th century Germany, through the lens of which he saw events in republican Rome.
For the convenience of those interested and/or participating in the upcoming book cat, I have created a PDF file of the section on Catlina.
