The missing portion of the preface will have contained, at least, an address to the dedicatee of the work, probably Cicero's brother Quintus, and an announcement of the subject, doubtless with some reference to Cicero's own position at the time of writing (the preface to De Oratore may be read for comparison).
Abstention from public life was recommended principally by the Epicureans, though this was subject to qualification (1. 10 below), and in fact many members of the Roman ruling class combined an interest in Epicurean philosophy with an active political career. At the point where our text begins, Cicero is using a familiar argument from the exploits of Roman patriots, a type of argument which he employed also in different philosophical contexts ( Paradoxa 1. 12, Tusculan Disputations 1. 39, Cato Maior75).
3the two Scipios: Gnaeus and Publius: the latter was the father of Scipio Africanus the elder.
Publius Africanus: Scipio Africanus the elder.
Cato: Cato the Elder, for whom Cicero had particular admiration as a 'new man' like himself; cf. R. 2. 1. Cato remained active in politics until his death in 149 BC, at the age of 85. His political career was indeed stormy; he is said to have survived 44 prosecutions.
Tusculum: now Frascati. This Latin community had received full Roman citizenship in 323 BC; cf. L 2. 5.
4moral excellence (virtus): see Note on the Translation. Cicero here touches on a question that was much discussed in ancient ethics; Socrates in Plato's dialogues constantly draws the analogy between moral virtues and practical skills or branches of knowledge (such as medicine or carpentry), and the Stoics, taking over the Socratic principle that virtue is a kind of knowledge, maintained that the truly wise and virtuous man was good at everything, including politics, regardless of whether he ever put his knowledge into practice. Cicero here maintains the common-sense (and Peripatetic) view that only the practical display of virtue qualifies a person to be called morally excellent.
most important field of practice . . . is in the government of a state: cf. R. 6. 13 (in the Dream of Scipio).