Friendship is a complicated concept in the world of the Late Republic. In fact, Cicero wrote an entire philosophical treatise on it (called de Amicitia, "on Friendship"). Cicero continually frames his relationship with Caesar as a friendship, and frequently writes to him and -of- him in warm tones.

Here is an excerpt from a letter written by Cicero to Caesar in mid-March of 49 BCE, two months after Caesar had invaded and chased Pompey and the majority of the Senate from Italy:

 

On reading your letter, handed to me by our friend Furnius, in which you ask me to come to the city walls, I was not so much surprised at your wishing "to avail yourself of my advice and position," but what you meant by speaking of my "influence and assistance" I did ask myself. My thoughts, however, were so far dominated by my hope, that I was induced to think that you wished to consult for the tranquillity, peace, and harmony of our fellow citizens: and for a policy of that kind I regarded both my natural disposition and my public character as sufficiently well adapted. If this is the case, and if you are at all anxious to preserve our common friend Pompey, and to reconcile him to yourself and the Republic, you will assuredly find no one better calculated than myself for supporting such measures. For, as soon as opportunity offered, I pleaded for peace both to him and the senate; nor since the commencement of hostilities have I taken any part whatever in the war; and I have held the opinion that by that war you are being wronged, in that men who were hostile to and jealous of you were striving to prevent your enjoying an office granted you by the favour of the Roman people. [Note] But as at that period I was not only personally a supporter of your rights, but also advised everybody else to assist you, so at the present moment I am strongly moved by consideration for the position of Pompey. It is now a good number of years ago since I picked out you two as the special objects of my political devotion, and—as you still are of my warm personal affection. Wherefore I ask you, or rather entreat you, and appeal to you with every form of prayer, that in the midst of your very great preoccupations you would yet spare some part of your time to reflect how by your kindness I may be enabled to do what goodness and gratitude, and, in point of fact, natural affection demand, by remembering the extreme obligation under which I stand. (ad Att. 9.11A)

 

As you can probably detect, this letter is obsequious in the extreme, dripping with stiff sentiment. This is a political friendship, a formalized stance, a public declaration of good will (or maybe, more properly, public lack of ill will). Caesar holds all the cards, and Cicero must play the game of friendship now as if nothing has happened. He absolutely despises the actions of Caesar in the civil war so far, and has said so to his friend Atticus in no uncertain terms:

 

Are we talking of an imperator of the Roman people, or of a Hannibal? [Caesar] is a madman! That miserable wretch, who has never seen even a shadow of virtue! And he says that he is doing all this "to support his honour"! How can there be any "honour" where there is no moral right? Can it be morally right to have an army without commission from the state? To seize cities inhabited by one's fellow citizens, as a means of attacking one's own country? To be contriving abolition of debts, restoration of exiles, hundreds of other crimes... (ad Att. 7.11).

 

On display here are two shades of "friendship" in the world of the Late Republican Roman elite. Cicero and Caesar were public friends, fellow aristocratic Romans, but they were not chums, not mates. They did not hang out at the taverna, just the two of them; when they had dinner together, it was a stiff and formalized affair. On the other hand, Cicero and Atticus were true friends. In fact, they were so close that Cicero felt safe in venting his above opinions about a man who had every semblance of becoming the next bloody Roman dictator. Sulla had killed men for less than the words written here, and Cicero is taking a very real risk in putting them to paper and sending them to a trusted friend in Rome.

Were Cicero and Caesar rivals? This is not easy to answer, but I would say that in hindsight, they were not. Cicero was at his most powerful during his consulship of 63, when he used his oratorical skill to ferret out the Catilinarian conspiracy. But it was not Cicero who led the Roman Republican army against the rebels in Etruria--he stayed home in Rome. He was not a military man, and though ambitious, his goals were not aligned with his elite Roman peers in this era (Pompey, Caesar, Crassus, Lucullus, e.g.). Cicero cooperated with Caesar -and- Pompey in the late 60s BCE politically, and was invited by them to participate in the cabal later known as the "first triumvirate." He declined. The triumvirate represented the willingness of those men to reach beyond the bounds of the Republican tradition, to take for themselves powers and authority beyond that which was "legal," and Cicero nowhere shows interest in such things. In this way he could never be a true rival to men like Caesar: he was permanently handicapped by his adherence to the mos maiorum, the conservative tradition of the Roman state, and its legal precedent.