Robert Stewart, 2nd Marquess of Londonderry, KG, GCH, PC, PC (Ire) (June 18, 1769 – August 12, 1822), usually known as Lord Castlereagh), is an Irish/British statesman.

As British Foreign Secretary, from 1812 he is central to the management of the coalition that defeats Napoleon and is the principal British diplomat at the Congress of Vienna.

Castlereagh is also leader of the British House of Commons in the Liverpool government from 1812 until his suicide in August 1822.

Early in his career, as Chief Secretary for Ireland, he is involved in putting down the Irish Rebellion of 1798 and is instrumental in securing the passage of the Irish Act of Union of 1800.

Castlereagh's challenge at the foreign office is to organize and finance an alliance to destroy Napoleon

He successfully brings Napoleon's enemies together at the Treaty of Chaumont in 1814.

Hereafter he works with Europe's leaders at the Congress of Vienna to provide a peace consistent with the conservative mood of the day.

At Vienna he is largely successful in his primary goal of creating a peace settlement that will endure for years.

He sees that a harsh treaty based on vengeance and retaliation against France will fail, and anyway the conservative Bourbons are back in power.

He employs his diplomatic skills to block harsh terms.

He holds the Chaumont allies together, most notably in their determination to finally end Napoleon's 100 Days in 1815.

He has a vision of long-term peace in Europe the united efforts of the great powers.

At the same time he is watchful of Britain's mercantile and imperial interests.

He purchases the Cape Colony and Ceylon from the Netherlands.

France's colonies are returned, but France has to give up all its gains in Europe after 1791.

He also works to abolish the international slave trade.

He is unsuccessful in avoiding the War of 1812 with the United States; it ends in a stalemate in 1814, with no boundary changes.

After 1815 Castlereagh is the leader in imposing repressive measures at home.

He is hated for his harsh attacks on liberty and reform.

However, in 1919 diplomatic historians will recommend his wise policies of 1814-1815 to the British delegation to the peace conferences that ends the First World War.

Historian Charles Webster underscores the paradox:
    There probably never was a statesman whose ideas were so right and whose attitude to public opinion was so wrong. Such disparity between the grasp of ends and the understanding of means amounts to a failure in statesmanship.