The 1240s was a decade of the Julian Calendar which began on January 1, 1240, and ended on December 31, 1249.
- December 6 – Batu Khan and the Golden Horde sack the Ruthenian city of Kiev.
- Tuan Mash'ika, an Arab, travels and introduces Islam to Sulu.
- July 15 – Battle of the Neva: Russian prince Alexander Nevsky defeats the Swedes, saving the Novgorod Republic from a full-scale enemy invasion from the North.
- The civil war era in Norway ends.
- The Flemish village Kaprijke is recognized as a city.
- Sancho II of Portugal conquers the cities of Ayamonte and Cacella from the Muslims, as part of the Reconquista.
- June 12 – The Disputation of Paris begins at the court of Louis IX of France, where four rabbis defend the Talmud against Nicholas Donin's accusations of blasphemy.
- Saint Maurice starts to be portrayed as a Moor.
- March 18 – Battle of Chmielnik (Mongol invasion of Poland): The Mongols overwhelm the feudal Polish armies of Sandomierz and Kraków provinces, and plunder the abandoned city of Kraków.
- April 9 – Battle of Legnica: The Mongols, under the command of Baidar, Kadan and Orda Khan, defeat the feudal Polish nobility, including the Knights Templar.
- April 11 – Battle of Mohi: Batu Khan and Subutai defeat Béla IV of Hungary. The battle is the last major event in the Mongol Invasion of Europe.
- April 27 – Battle of Sajo: The Mongols defeat Bela IV of Hungary.
- May – Battle of Giglio: an Imperial fleet defeats a Genoan fleet in the Tyrrhenian Sea.
- May 10 – Battle of Cameirge in Ulster: The Milesian Irish septs of the Ó Dónaills from Donegal, the Ó Néills from Armagh and the Ó Dochartaighs of Connacht defeat the last Tuatha Dé Danann sept, the Meic Lochlainn of Tír Eoghain and Inishowen under Domhnall mac Muirchertaigh Mac Lochlainn. From now on the Kings of Tír Eoghain will all be of the Ó Néill dynasty, Brian Ua Néill becoming sole ruler.
- Early northern summer – A succession crisis or other priorities results in the Mongols withdrawing behind their river barrier into the Ukraine and the Russias, leaving Central Asian and far Eastern Europe peoples tributary to the Khanates, but leaving Poland and Hungary to begin recovery and reorganization.
- August 29 – After Henry III of England's invasion of Wales, the Treaty of Gwerneigron is signed by him and Dafydd ap Llywelyn, curbing the latter's authority and denying him royal title.
- September 23 – Snorri Sturluson, Icelandic saga writer, is murdered by Gissur Þorvaldsson, an emissary of King Haakon IV of Norway.
- October 25 – Pope Celestine IV succeeds Pope Gregory IX, as the 179th pope.
- Emperor Lizong of Song China accepts the Neo-Confucian teachings of the late Zhu Xi, including his commentary on the Four Books. This will have an impact upon the philosophical schools of surrounding countries as well, including Korea, Japan, and Vietnam.
- Livonian Crusade: The Estonian rebellion of 1237 is suppressed on Saaremaa Island, by the Livonian Order.
- In Maghrib, after a string of successes against the fast declining Almohads, Abu Zakariya, the first Hafsid ruler of Ifriqiya, conquers the Kingdom of Tlemcen.
- Emperor Go-Saga ascends to the throne of Japan.
- Batu Khan establishes the Golden Horde at Sarai.
- The Mongols invade the Seljuk Sultanate.
- April 5 – During a battle on the ice of Lake Peipus, Russian forces, led by Alexander Nevsky, rebuff an invasion attempt by the Teutonic Knights.
- Cleves, Germany is chartered as a city.
- Kiel, Germany is chartered as a town.
- The Archbishop of Mainz conquers the city of Wiesbaden, from the House of Nassau.
- King Sancho II of Portugal conquers the cities of Tavira, Alvor and Paderne, in his continuing effort against the Muslims, known as Reconquista.
- Mongol invasions
- German colonists arrive in Bratislava, after the Mongols fail to conquer the city.
- The Mongols of the Golden Horde devastate Volga Bulgaria, and force the nation to pay tribute.
- A French goldsmith working in Budapest, named Guillaume Boucher, is captured by the Mongols and taken to Karakorum.
- The Golden Bull is issued by King Béla IV to inhabitants of Gradec (today's Zagreb) and Samobor in Croatia, during the Mongol invasion of Europe. By this golden bull King Bela IV proclaim a free royal city.
- Battle of Grobnicko Polje: Croats stop the Mongolian invasion.
- Timeline of medicine and medical technology: Ibn Nafis suggests that the right and left ventricles of the heart are separate, and describes the lesser circulation of blood.
- The diocese of Warmia, Poland is created.
- March – Treaty of Alcaraz: Ferdinand III of Castile turns the independent Muslim Taifa of Murcia into a protectorate, and initiates the process of the colonization and Christianization of the region.
- May 1 – The Castillan troops are garrisoned in Murcia, to support the Huddite Dynasty.
- June 25 – Pope Innocent IV succeeds Pope Celestine IV, as the 180th pope.
- June 26 – Battle of Köse Dağ: The Mongols defeat the Seljuk Turks of the Sultanate of Rum.
- The city of Brno is founded, in what will become the Czech Republic.
- March 16 – Following their successful nine-month siege of Montségur, French royal forces burn about 210 Cathar Perfecti and credentes.
- The Christian Reconquista in Iberia enjoys a string of successes:
- March 26 – By the Treaty of Almizra, the king of Aragon and prince of Castile come to an agreement, on the distribution of Muslim lands yet to be conquered.
- May 22 – James I of Aragon takes the Muslim-held city of Janita, after several months of siege. This success is followed by the capture of Biar later that year.
- James I of Aragon reconquers Altea, Spain.
- The heir prince of Castile conducts a series of military operations, to support the Muslim Huddite rulers of Murcia against rebel strongholds.
- The Siege and Fall of Jerusalem leads to the Seventh Crusade.
- Dogen Zenji establishes the Eiheiji temple in Japan, thus founding the Sōtō sect of Zen Buddhism.
- February 21 – Thomas (bishop of Finland) is granted resignation by Pope Innocent IV, after having confessed to torture and forgery.
- April 16 – Pope Innocent IV sends Giovanni da Pian del Carpine to the Mongol court, suggesting (amongst other things) that the Mongols convert to Christianity, and join the Crusades.
- June 28 – The First Council of Lyon opens, in the course of which Frederick II, Holy Roman Emperor, is excommunicated and deposed, and the Seventh Crusade is proclaimed.
- August 1 – The second of two papal bulls refers to the marriage of King Sancho II of Portugal to Mécia Lopes de Haro, and decrees the deposition of the king.
- date unknown
- Emperor Go-Fukakusa succeeds Emperor Go-Saga, on the throne of Japan.
- Güyük Khan is enthroned as the 3rd Great Khan of the Mongol Empire (an event also witnessed by a papal mission under Giovanni da Pian del Carpine), at Karakorum.
- With the death of Duke Frederick the Quarrelsome, the Babenberg Dynasty ends in Austria.
- Spain: After two unsuccessful sieges in 1225 and 1230, the Castillans manage to take the city of Jaén from the Andalucians, at the Siege of Jaen.
- The Gothic chapel of Sainte-Chapelle is built.
- Robert Grosseteste translates Aristotle's Nicomachean Ethics from Greek into Latin, which marks the true start of the rediscovery of the philosopher by medieval Europe.
- Beaulieu Abbey is dedicated.
- July 2 – A document issued by King Béla IV of Hungary, granting territories to the Knights Hospitaller in the Banate of Severin and Cumania, makes an early mention of Litovoi and other Vlach/Romanian local rulers, in Wallachia and Transylvania.
- December 1 – A rebellion arises among the Muslim subjects of the Crown of Aragon, in the region of Valencia. As a punishment, the king issues an order of expulsion of the Muslims from his realm, leading numerous people into exile in Andalusia and North Africa, in the subsequent year.
- Shams Tabrizi disappears, resulting in Jalal Uddin Rumi writing 30,000 verses of poetry about his disappearance.
- Romford, London, England is chartered as a market town.
- The future Bethlem Royal Hospital, Bedlam, is founded in London.
- The Thuringian War of Succession begins.
- Qin Jiushao publishes the original form of the Chinese remainder theorem.
- Egypt takes control of Jerusalem from the Kharezmians.
- Nijmegen becomes part of Gelderland.
- Afonso III succeeds Sancho II, as King of Portugal.
- Song Ci publishes the Collected Cases of Injustice Rectified, a book considered to be the first monographic work on forensic medicine.
- April 26 – The Gothic chapel Sainte-Chapelle is consecrated in Paris, France.
- August 15 – The foundation stone of Cologne Cathedral is laid, after an older cathedral on the site burns down on April 30 (construction is completed 632 years later, in 1880).
- August 25 – The Dutch city of Ommen receives city rights and fortification rights from Otto III, Archbishop of Utrecht, after it has been pillaged at least twice by a local robber baron.
- November 23 – Reconquista: King Ferdinand III of Castile recaptures the city of Seville from the Moors, ending the Siege of Seville; this year also Prince Alfonso X of Castile takes the city of Alicante.
- November 24 – In the middle of the night a mass on the north side of Mont Granier suddenly collapses, in one of the largest historical rock slope failures known in Europe.
- King Louis IX of France launches the Seventh Crusade, setting sail with an army of 20,000 toward Egypt.
- Pope Innocent IV grants the Croats permission to use their own language and script in liturgy (see Glagolitic alphabet).
- Tallinn (Reval) converts from Riga law to Lübeck law.
- Roger Bacon publishes the formula for black powder in Europe.
- Approximate date – History of the Aztecs: The Mexica tribe, predecessors of the Aztec people, arrive at Chapultepec (in modern-day Mexico City).
- Tezcapoctzin becomes Ruler of the City-state Azcapotzalco at the Valley of Mexico
- King Louis IX of France captures Damietta in Egypt, in the first major military engagement of the Seventh Crusade.
- Pho Khun Si Indrathit becomes the first king of the Sukhothai Kingdom, marking the founding of the modern Thai nation.
- The Hikitsuke, a judicial organ of the Kamakura and Muromachi shogunates of Japan, is established.
- The Japanese Hōji era ends, and the Kenchō era begins.
- February 16 – Andrew of Longjumeau is dispatched by King Louis IX of France as an ambassador, to meet with the Khan of the Mongols.
- May 26 – The Battle of Fossalta is fought between the Holy Roman Empire and the Lombard League. The Italians capture the German commander.
- July 13 – Alexander III is crowned as King of the Scots.
- August 15 – The First Battle of Athenry is fought in Galway, Ireland.
- The city of Stralsund (in present-day Germany) is burned to the ground, by forces from the rival city of Lübeck.
- Swedish statesman Birger Jarl subjugates the province of Tavastia in Finland, securing Swedish power in Finland.
- Alphonse, Count of Poitiers orders the expulsion of Jews from Poitou, France.
- The Hungarian capital is moved from Esztergom to Buda.
- The Moors lose possession of Alicante in Al-Andalus (Islamic Spain).
- King Afonso III of Portugal recaptures Faro and Silves in the Algarve from the Moors, thus ending the Portuguese Reconquista.
- The city of Mystras, Greece is fortified, and a palace is constructed there by William II Villehardouin.
- Spring – University College, the first College at Oxford, is founded with money from the estate of William of Durham.
- Jean Mouflet makes an agreement with the abbot of Saint-Pierre-le-Vif in the Senonais region in France: in return for an annual payment, the monastery will recognize Jean as a "citizen of Sens". He is a leather merchant, with a leather shop that he leases for the rent of 50 shillings a year. The agreement is witnessed by Jean's wife, Douce, daughter of a wealthy and prominent citizen of Sens, Felis Charpentier.
Fibonacci. Fibonacci sequence and Liber Abbaci
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