1840s

thumb|335x335px|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.]]

The 1840s (pronounced "eighteen-forties") was the decade that began on January 1, 1840, and ended on December 31, 1849.