September 17

Seljuk Turks defeat Byzantine forces at the Battle of Myriokephalon, ending Byzantine hopes of recovering central Anatolia.

Byzantine Emperor Manuel I Komnenos led an army into Anatolia attempting to reassert control over former Byzantine territory but suffered a devastating ambush by Seljuk Turkish forces at Myriokephalon, a defeat that permanently ended meaningful Byzantine hopes of reclaiming the Anatolian heartland from Turkish control.

Polish forces defeat the Teutonic Order at the Battle of Swiecino.

Polish and allied forces decisively defeated the Teutonic Order near the village of Swiecino in Pomerania during the Thirteen Years' War, a prolonged conflict fought over control of the Baltic coastal region. The defeat ultimately weakened the once-powerful crusading military order's territorial holdings and financial resources, contributing to Poland's eventual gains under the subsequent peace settlement that concluded the war.

The Abckiria, the first book published in the Finnish language, is printed in Stockholm.

Bishop Mikael Agricola's translation of a basic primer combining the alphabet, prayers, and catechism became the first book ever printed in the Finnish language, a foundational milestone in establishing Finnish as a written literary language distinct from Swedish, the region's dominant administrative tongue.

The city of Boston, Massachusetts, is founded by Puritan settlers.

Puritan colonists led by John Winthrop established a settlement on the Shawmut Peninsula that they named Boston, after a town in Lincolnshire, England, from which several early settlers had emigrated. The settlement rapidly grew into one of colonial America's most influential religious, intellectual, and commercial centers, later playing a central role in the events leading up to the American Revolution.

Delegates to the Constitutional Convention sign the United States Constitution in Philadelphia.

After a demanding summer of debate and compromise among delegates representing the thirteen states, the Constitutional Convention concluded with 39 delegates signing the newly drafted United States Constitution in Philadelphia, establishing the fundamental framework of American federal government still in use today.

The Battle of Antietam is fought in Maryland, remaining the bloodiest single day in American military history.

Union and Confederate forces clashed along Antietam Creek in Maryland in a single day of fighting that produced roughly 23,000 combined casualties, the bloodiest day in American military history, a costly tactical draw that nonetheless gave President Lincoln the political opening to issue the Emancipation Proclamation.

The National Football League is organized in Canton, Ohio.

Representatives from several professional football clubs met in Canton, Ohio, to establish the American Professional Football Association, which was renamed the National Football League two years later, laying the foundation for what became the most-watched professional sports league in the United States.

The Soviet Union invades Poland from the east, weeks after Germany's invasion from the west.

Fulfilling the secret provisions of the Molotov-Ribbentrop Pact signed with Germany weeks earlier, Soviet forces invaded Poland from the east, completing the country's partition between the two totalitarian powers and effectively eliminating Poland as an independent state for the remainder of the war.

The Polish trade union Solidarity is formally established following strikes at the Lenin Shipyard in Gdansk.

Following widespread strikes protesting economic conditions and demanding independent labor representation, the Solidarity trade union was formally established at the Lenin Shipyard in Gdansk, becoming the first independent labor union recognized within the Soviet-aligned Eastern Bloc and a major catalyst for the eventual collapse of Communist rule in Poland.

The Occupy Wall Street movement begins in Zuccotti Park, New York City.

Protesters against economic inequality and corporate influence in politics established an encampment in Manhattan's Zuccotti Park, launching the Occupy Wall Street movement, which popularized the phrase "We are the 99 percent" and inspired similar protest movements in cities around the world.