This is from Macaulay's essay on Hallam's history.
"The effect of violent animosities between parties has always been an indifference to the general welfare and honour of the State. A politician, where factions run high, is interested not for the whole people, but for his own section of it. The rest are, in his view, strangers, enemies, or rather pirates. The strongest aversion which he can feel to any foreign power is the ardour of friendship, when compared with the loathing which he entertains towards those domestic foes with whom he is cooped up in a narrow space, with whom he lives in a constant interchange of petty injuries and insults, and from whom, in the day of their success, he has to expect severities far beyond any that a conqueror from a distant country would inflict. . . . In the Italian states of the thirteenth and fourteenth centuries, from the same cause, no man was so much a Pisan or a Florentine as a Ghibeline or a Guelf. It may be doubted whether there was a single individual who would have scrupled to raise his party from a state of depression, by opening the gates of his native city to a French or an Arragonese force. The Reformation, dividing almost every European country into two parts, produced similar effects. The Catholic was too strong for the Englishman, the Huguenot for the Frenchman. The Protestant statesmen of Scotland and France called in the aid of Elizabeth; and the Papists of the League brought a Spanish army into the very heart of France. The commotions to which the French Revolution gave rise were followed by the same consequences. The Republicans in every part of Europe were eager to see the armies of the National Convention and the Directory appear among them, and exalted in defeats which distressed and humbled those whom they considered as their worst enemies, their own rulers. The princes and nobles of France, on the other hand, did their utmost to bring foreign invaders to Paris. A very short time has elapsed since the Apostolical party in Spain invoked, too successfully, the support of strangers."