Niccolò Machiavelli
Discourses on the First Decade of Titus Livius (Chap. 1.19) (English version)
After a strong Prince a weak Prince may maintain himself: but after one weak Prince no Kingdom can stand a second.

When we contemplate the excellent qualities of Romulus, Numa, and Tullus, the first three kings of Rome, and note the methods which they followed, we recognize the extreme good fortune of that city in having her first king fierce and warlike, her second peaceful and religious, and her third, like the first, of a high spirit and more disposed to war than to peace. For it was essential for Rome that almost at the outset of her career, a ruler should be found to lay the foundations of her civil life; but, after that had been done, it was necessary that her rulers should return to the virtues of Romulus, since otherwise the city must have grown feeble, and become a prey to her neighbours.

And here we may note that a prince who succeeds to another of superior valour, may reign on by virtue of his predecessor's merits, and reap the fruits of his labours; but if he live to a great age, or if he be followed by another who is wanting in the qualities of the first, that then the kingdom must necessarily dwindle. Conversely, when two consecutive princes are of rare excellence, we commonly find them achieving results which win for them enduring renown. David, for example, not only surpassed in learning and judgment, but was so valiant in arms that, after conquering and subduing all his neighbours, he left to his young son Solomon a tranquil State, which the latter, though unskilled in the arts of war, could maintain by the arts of peace, and thus happily enjoy the inheritance of his father's valour. But Solomon could not transmit this inheritance to his son Rehoboam, who neither resembling his grandfather in valour, nor his father in good fortune, with difficulty made good his right to a sixth part of the kingdom. In like manner Bajazet, sultan of the Turks, though a man of peace rather than of war, was able to enjoy the labours of Mahomet his father, who, like David, having subdued his neighbours, left his son a kingdom so safely established that it could easily be retained by him by peaceful arts. But had Selim, son to Bajazet, been like his father, and not like his grandfather, the Turkish monarchy must have been overthrown; as it is, he seems likely to outdo the fame of his grandsire.

I affirm it to be proved by these examples, that after a valiant prince a feeble prince may maintain himself; but that no kingdom can stand when two feeble princes follow in succession, unless, as in the case of France, it be supported by its ancient ordinances. By feeble princes, I mean such as are not valiant in war. And, to put the matter shortly, it may be said, that the great valour of Romulus left Numa a period of many years within which to govern Rome by peaceful arts; that after Numa came Tullus, who renewed by his courage the fame of Romulus; and that he in turn was succeeded by Ancus, a prince so gifted by nature that he could equally avail himself of the methods of peace or war; who setting himself at first to pursue the former, when he found that his neighbours judged him to be effeminate, and therefore held him in slight esteem, understood that to preserve Rome he must resort to arms and resemble Romulus rather than Numa. From whose example every ruler of a State may learn that a prince like Numa will hold or lose his power according as fortune and circumstances befriend him; but that the prince who resembles Romulus, and like him is fortified with foresight and arms, will hold his State whatever befall, unless deprived of it by some stubborn and irresistible force. For we may reckon with certainty that if Rome had not had for her third king one who knew how to restore her credit by deeds of valour, she could not, or at any rate not without great difficulty, have afterwards held her ground, nor could ever have achieved the great exploits she did.

And for these reasons Rome, while she lived under her kings, was in constant danger of destruction through a king who might be weak or bad.