1840s

From top left, clockwise: The Mexican–American War ushers in the American expansion in its western frontier, paving way for new territories (and eventually states) such as Texas and California; the signing of the Treaty of Waitangi in 1840 guarantees continued Māori sovereignty but also leads to the proclamation of the nominal Colony of New Zealand; The great auk goes extinct, as it falls victim to overhunting; The First Opium War catalyzes Europe's imperial encroachment and control over Chinese ports, as the war resulted with Hong Kong's succession to Britain via the Treaty of Nanking; The Oregon Trail opens up to the world, prompting a wave of migration to the American west and later on, a gold rush in California that persisted through the 1850s; The saxophone is patented, later used in jazz, swing, and blues; First edition of the Communist Manifesto is published by Karl Marx in February 1848, and goes on to create a revolutionary shift in political ideologies and thought in the 20th century, influencing entire states such as Soviet Union, China, and Cuba; the Revolutions of 1848 ravages European politics, and causes multiple socio-cultural changes, particularly in classical music, arts, and politics.
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The 1840s (pronounced "eighteen-forties") was a decade of the Gregorian calendar that began on January 1, 1840, and ended on December 31, 1849.

The decade was noted in Europe for featuring the largely unsuccessful Revolutions of 1848, also known as the Springtime of Nations. Throughout the continent, bourgeois liberals and working-class radicals engaged in a series of revolts in favor of social reform. In the United Kingdom, this notably manifested itself through the Chartist movement, which sought universal suffrage and parliamentary reform. In France, the February Revolution led to the overthrow of the Orléans dynasty by Louis-Napoleon Bonaparte. In 1848, the publication of the Communist Manifesto by Karl Marx would help lay the groundwork for the global socialist movement. Arguably the first major event of the decade was the signing of the Treaty of Waitangi in the United Tribes (modern-day New Zealand) between Māori rangatira and representatives of the British Crown, which began in February 1840. Due to the differences between the Māori and English versions of the texts, the British claimed Māori had ceded sovereignty and proclaimed a new Colony, leading to more than 25 years of asymmetric armed conflict until the Colony secured substantive control.

The Mexican–American War led to the redrawing of national boundaries in North America. In the United States, mass migration to the new West Coast occurred, following the annexation of California from Mexico with the Treaty of Guadalupe Hidalgo, and the California Gold Rush beginning, following the discovery of gold there, both in early 1848. On its northern border, the United States settled the Oregon boundary dispute with the United Kingdom in 1846, thereby solving a domestic political crisis in the former nation. Meanwhile in Ireland, the Great Famine began in 1845, causing the deaths of one million Irish people and forcing over a million more to emigrate. In 1848, the women's rights movement began with the Seneca Falls Convention in New York.

The last living person from this decade was Robert Early, who died in 1960.