One of the things I found most interesting about Castlereagh’s post-1815 direction of British foreign policy was his recognition that the continental threat to Britain ended with the defeat of Napoleon and that no new threats needed to be invented to replace it. Having concluded a long and costly struggle, Castlereagh saw no reason to embark on another one, and his hawkishness in fighting Napoleon didn’t translate into an obsession with waging new wars that weren’t necessary for British security. As I put it in my review of John Bew’s Castlereagh: A Life:

The principle of nonintervention that Castlereagh outlined in his State Paper of May 5, 1820 reflected his thinking at that point in the postwar period. It rejected the option of supporting the re-establishment of monarchical governments in Spain and Portugal and reaffirmed that Britain’s interest was in collective European peace and security. As the paper put it, Britain did not belong to an alliance “intended as an Union for the Government of the World, or for the superintendence of the internal affairs of other States.” Having prevailed over Napoleon, Britain under Castlereagh’s guidance was not bent on an ideological project of restoration, and it had nothing at stake in internal political conflicts elsewhere in Europe.

Unlike some of the monarchs and ministers in other European states, he had no desire to commit Britain to a new series of conflicts for the sake of restoring monarchies where they had been overthrown. Though he became a close colleague and friend of Pitt and served in Tory governments throughout his career, he was in some respects less fervently counter-revolutionary than his fellow Irishman Burke, and this was something he had exhibited early on in his public career. For example, Burke was an early advocate of military intervention in support of restoring the French monarchy, which Castlereagh thought to be unwise in the 1790s. Bew describes Castlereagh’s view:

Arguing directly against Burke, in fact, Castlereagh strongly opposed calls for a counter-revolutionary army to be sent to France to restore the Bourbon regime. He was well aware that the sovereigns of Europe were ‘trembling’ as they watched events in Paris and were eager ‘to subdue that spirit which may overwhelm them’. However, he warned that they should ‘cautiously weigh the practicality of such an attempt’ and to consider ‘whether it is possible at present to extinguish the flame’. Furthermore, any counter-revolutionary regime which ‘received its power from foreign interference, in a kingdom such as France, too important to be dictated to [bold mine-DL], will hold it by an uncertain tenure’. (p. 47-48)

One of the things that many interventionists today consistently underestimate is the degree to which other nations resent foreign interference in their affairs regardless of the side in their internal disputes that the interference benefits. On this question, Castelreagh’s judgment was better than Burke’s. Towards the end of his career, Castlereagh was even more adamant that Britain would not try to manage the internal political developments of other states. Bew explains:

Thus, while Castlereagh made it clear that his insistence on non-intervention was ‘not absolute’–and that Britain would be found in her place ‘when actual danger menaces the System of Europe’–he was emphatically clear that ‘this Country cannot, and will not, act upon the abstract and speculative principles of Precaution’. It was apparent that many European states were ‘now employed in the difficult task of casting anew their Govts. upon the Representative Principle: but the notion of revising, limiting or regulating the course of such Experiments, either by foreign Council or foreign foe, would be as dangerous as to avow as it w[ould] be impossible to execute, and the Illusion too prevalent on this Subject, should not be encouraged in our Intercourse with the Allies’. (p. 482)