Bew goes very much against the current historical trend, at least in the academy, of regarding the Toryism of the day as all about the Church of England and evangelical Protestantism. The Established Church meant little to Castlereagh, who saw his conservatism as merely a common-sense creed, partly forged out of the need to win the Revolutionary and Napoleonic Wars. Bew shows how it was no coincidence that (with the exception of Wellington) the strongest supporters of Catholic Emancipation in the government were also the strongest hawks. They wanted domestic tranquillity and the minimum possible religious tension for the Jacobins to exploit. It was for stances such as this that Lord Salisbury described Castlereagh as "a practical man of the highest order, who yet did not by that fact forfeit his title to be considered a man of genius".  

Radicals never forgave Castlereagh for his roles in thwarting the intended French invasion of Ireland in 1796, which they (wrongly) saw as an attempt at liberation, then for crushing the 1798 Rebellion there, and finally passing the Act of Union in 1800, which led to 121 years of legislative union and the dawn of our United Kingdom. The political unity of the British Isles kept Ireland free of a Napoleonic rule that would have been far worse than that of Britain, and set the scene for the huge expansion of the British Empire — especially once Castlereagh had secured key imperial nodal points at the Congress of Vienna — which also hugely benefited his countrymen. Although Castlereagh supported Catholic Emancipation, he is given scant credit for that by his detractors.

Castlereagh spent most of his adult life in government. An admirer and faithful follower of William Pitt the Younger, he was President of the Board of Control from 1802 to 1806 and appointed by Pitt to the War and Colonial Office in 1805; indeed, it was in his anteroom that Nelson and Wellington met for the only time. As War Secretary he secured the Danish fleet and saved the Swedish and Portuguese fleets from being captured by Napoleon. In 1808 he sent Wellington to the Peninsula (against George III's wishes) and supported him through the court-martial and obloquy that followed the Convention of Cintra. When in 1809 Castlereagh discovered that the Foreign Secretary, George Canning, had been conspiring against him, he fought a duel on Putney Heath and shot him in the thigh. (After both men had missed at ten paces, he had demanded a second shot. Canning behaved slightly better thereafter.)  

Resigning after the duel he was back in office in 1812 as Foreign Secretary, and also became leader of the House of Commons until he succeeded to his father's marquessate in 1821, although he carried on at the  Foreign Office until his suicide on August 12, 1822. His period as Foreign Secretary saw Britain concluding treaties with Russia, Austria, Turkey, Sweden and Russia; mediating peace between former enemies; massively increasing subsidies to countries willing to fight Napoleon; arranging the affairs of Italy with Austria; signing the Treaty of Chaumont and installing representative government in France after Napoleon's first abdication; abolishing the slave trade in France, Belgium and Spain; refusing a separate peace when Napoleon escaped from Elba, and sending him to St Helena after Waterloo; and securing a peace at Vienna in 1815 that was to last until the Crimean War in 1854. It was a stupendous achievement, often in the face of wildly histrionic attacks from the Whigs and Radicals.