In the year AD 1621...
- The Twelve Years' Truce between the Dutch Republic and the Spanish Empire expires, and both sides prepare to resume the Eighty Years' War.
- Meanwhile, the Thirty Years' War enters its fourth year.
- After the debacle at White Mountain the year before, the Protestant cause looks lost, and the Protestant Union formally disbands, but a few remnant Protestant forces remain in the field.
- The largely unscathed mercenary army under Ernst von Mansfield retreats to the Upper Palatinate.
- Meanwhile, remnants of the Bohemian army manage to hold off an Imperialist advance into Silesia for the time being, parrying their thrust at the Battle of Neu Titschein.
- In the fall, a new Protestant army is recruited in record time by 22-year-old adventurer Christian of Brunswick, using Dutch funds.
- As the year draws to a close, Holy Roman Emperor Ferdinand II shores up his eastern flank by negotiating the Peace of Nikolsburg with Transylvanian prince Gabor Bethlen. Bethlen renounces his claims to Hungary and agrees to stop supporting the Protestants, and in return is granted a few border counties and a promise of religious freedom in Moravia.
- The First Huguenot Rebellion continues in France, where forces led by French king Louis XIII besiege and capture Saint-Jean-d'Angély, a crucial fortress controlling the approach to the Huguenot stronghold at La Rochelle.
- In the Battle of Khotyn, Polish forces under Grand Hetman of Lithuania Jan Karol Chodkiewicz manage to hold off an army of invading Ottomans under Sultan Osman II until the onset of winter, leading to the signing of the Treaty of Khotyn and the end of the Polish-Ottoman War, although Chodkiewicz is killed in the fighting.
- Manchurian warlord Nurhaci continues to expand his empire, conquering the Liaodong Peninsula and Shenyang (Mukden).
- The Pilgrims of Plymouth Colony and Wampanoags celebrate a harvest feast, later regarded as the "First Thanksgiving."
- Swedish king Gustavus Adolphus founds the city of Gothenburg, which today is the second largest city in Sweden.
- Dutch astronomer and mathematician Willebrord Snellius mathematically derives the law of refraction of light now eponymously known as Snell's Law.
- Robert Burton publishes his satirical masterwork, The Anatomy of Melancholy.
These people were born in 1621...
These people died in 1621...