Karl Marx and Friedrich Engels publish in London a pamphlet setting out the theory of class struggle, one of the most influential political texts ever written.
A wave of revolutions sweeps Europe, from France to the German states, Austria, Italy and Hungary, demanding democratic reform and national self-determination.
Potato blight triggers a famine that kills about a million people and drives more than a million to emigrate from Ireland.
A series of armed conflicts between the colonial government and Māori iwi, chiefly over land and sovereignty, which led to large confiscations of Māori land.
An alliance of the Ottoman Empire, Britain, France and Sardinia defeats Russia, with fighting centred on the siege of Sevastopol in Crimea.
The Qing cede Hong Kong Island to Britain and open five treaty ports, the first of China's unequal treaties.
The physician John Snow links a London cholera outbreak to a public water pump, a founding moment of modern epidemiology.