The following chronologies show events between AC1 and AC2. I focus on the Levant, Egypt, Mongol Empire, France, Italy, England, and HRE to show some interesting links in the series. It can show some interesting link and referenece through the series
Light Blue= Historical event
Light Orange=Fictional event
Light Pink=Historical event but with a spin
Ayyubids and Crusader states conflict
After the Third Crusade, the Levant wasn't at peace. Each time Christians and Muslims stopped to fight, internal conflict appeared. The Sultanate Ayyubid was divided into three between Saladin sons (sultanate of Egypt, Damascus and Aleppo). Later Egypt and Aleppo became rivals. Each sultan allied with Crusaders to fight.
The Crusaders also fought for power. Conrad II of Jerusalem, son of William of Montferrat, was assassinated by the Brotherhood in 1192. His rival, Henry of Champagne, an ally of the Assassins, wedded his widow and became king. He died in 1197, falling from a balcony. The absence of a king led the lords and barons to fight.
Sicily
The emperor Frederick II could be an interesting ally for the Brotherhood. His father was the Emperor Henry VI. He imprisoned Richard Lionheart, accusing him of the murder of Conrad II. He invaded Sicily and killed the heir. Even his wife revolted against him. In 1197, he died while preparing a Crusade. Maybe he could be a Templar.
His son Frederick was only a child at his death and had as teacher pope Innocent III. He was well educated and had knowledge of Arab culture as Sicily was a crossroad between the Greek and the Arab world. In 1227 he went to Levant and became king of Jerusalem through diplomacy. He even sent a delegation to Masyaf and had scholars from Alamut in his court. Maybe he could have met Altaïr during his exile.
Mamluk vs Mongol
Historically, the Assassins existed as an independent state until the 1270s, even retaking Alamut in 1275. The Mamluks took most of their fortress from the Mongols and used the Assassins as personal hitmen. In the French comic (ambiguous canonicity), they said the Assassins supported the Mamluk revolt against the Ayyubids of Egypt. Maybe they allied to fight the Mongols who were involved with the Templars in the Levant.
In Memories, we learned that the Assassins orchestrated the Toluid war to weaken the empire. The empire was divided into four: the Yuan Dynasty in China, Ilkhanate in Persia and Levant, Changatai Khanate in Central Asia, and the Golden Horde in Eastern Europe. Maybe the Assassins participated in the Red Turbans rebellion in China and helped the Ming ascension.
The military monastic order legacy
After Acre was taken by the Mamluks in 1291, the Crusaders and the military order moved to Cyprus which became the base for future Crusade projects until 1489. The King of Cyprus was by title the King of Jerusalem. When the Templars were arrested in France, the king of Cyprus protected them from trial at first. The Hospitaliers conquered Rhodes from the Byzantines in 1309 and received the assets of the Templars in 1312.
On the Teutonic side, they went to Venice in 1291 before moving in 1309 to Marienburg in Poland. Since 1226, the Order controlled Prussia and participated in the Northern Crusades against heathens in Lithuania. The Teutonic first supported Poland but they began to conquer them. This led to the union between Poland and Lithuania in 1385. After defeating them, Teutonic Prussia became a vassal of Poland.
In Iberia, the Templars were protected and their assets financed new orders, the Order of Montesa in Spain, and the Military Order of Christ in Portugal with the King as their Grand Master. The latter participated in the Portuguese explorations. In Assassin's Creed, the Templars were involved in these journeys.
France and England
The link between France and the Assassins and France could be traced back to 1238 when a delegation of Assassins went to France to warn about the Mongols and search ally (it is a historical fact. Darim went to France and England to do that, maybe it is a reference). King Louis IX also met the Assassins in Acre in 1248. It could explain why the Sainte-Chapelle had the entrance of the Parisian Brotherhood headquarters.
For England, another interesting link. Edward I of England, when he was a prince in the Crusade, survived an assassination attempt by the Brotherhood. He later invaded Scotland and was manipulated by the Templars. William Wallace who led the Scottish rebellion, was an Assassin. Maybe his execution in 1305 was orchestrated by the Order. Some rumors also said that King Robert de Bruce was helped by Templars during the Battle of Bannockburn in 1314 (they were already arrested).
The Montfort House is also a recurring actor in European politics. The brothers Simon and Guy led the Albigensian Crusades against the Cathars in Southern France. They died during the conflict, the first in 1218 and the second ten years later. Both their sons had interesting stories. Simon V (Simon's son) led the Second Baron's War in England which formed the first parliament. Simon V's cousin, Philip was a Crusader and lord of Tyre. He participated in the War of the Lombards against Frederick II, the Seventh Crusade, and the War of Saint Sabas. He was assassinated by the Brotherhood in 1270, the last known official target. Maybe the Montfort are backed by the Templars.
We already know the implications of the two factions in the Hundred Years' War but it is interesting to see that they had a lot of assassinations before the Lancastrian War. Other interesting info, the Assassin René d'Anjou became king of Naples but lost his Kingdom in 1442 to Aragon.
The Francocratia and the Italian diaspora
After the sack of Constantinople, the Crusaders created the Eastern Latin Empire with duchies and kingdoms in Greece while the Byzantine retreat in Anatolia. They returned in 1261, but some Frank duchy stayed until the Ottoman conquest of Greece. The Duchy of Athens is an interesting case. It was conquered by Almogavars mercenaries who fought the Turks and the Greeks for them. Later the Acciaiuoli, a Florentine family close to the Medici, ruled until 1456, three years after the Fall of Constantinople. The Monferra were kings of Thessalonia and made a marital alliance with thePalaiologi.
The Italian diaspora was important, as were the Venetian, Genoese, and Pisans, who made trade routes and contact with the East and imported the Greek culture leading to the Renaissance. They often allied in naval battles against the Ottomans who spread from the Anatolian coasts to Greece. But Venice and Genoa were usually at war to control the Mediterranean Sea. Maybe the Italian Brotherhood sent agents during this period.