Unlike in OTL where the Western Roman Empire was the first to collapse, this timeline features the falling of the Eastern Roman Empire (Byzantium) rather than its western counterpart first. The most prominent long-term side effect of this divergence is the rapid, nearly complete unification of the globe through Islam.
Turning Point[]
Instead of killing competent generals like Aetius in the fourth century AD and embarking on costly appeasement with the Goths, the Romans instead developed a commonwealth system in the West. Migrating barbarians were given land, their territories ruled by traditional systems, but swore allegiance to Rome and provided men for their armies. In this way the barbarians who came into the system made the west stronger (but different). A commonwealth fought together in their common defence. Thus, the Empire of the west was more devolved, but continued to have a steady overall government.
Meanwhile, the Eastern Empire of Byzantium failed to adapt, and barbarians facing a strong western alliance moved east instead, taking land from the East. In the 5th century, a disastrous campaign against those tribes, plus the loss of the Syria-Judean provinces by a renewed Sassanid empire, meant the death knell for Byzantium, which only controlled the area now known as Turkey/Armenia.
Dark Ages[]
Due to Western Rome's survival, the Anglo-Saxon invasions of the British Isles (under Roman control) are successfully repelled and thus, the country of England never exists. Latin also survives longer and the languages of French, Italian, Spanish and Portuguese end up non-existent.
Islam, established in Arabia during the 7th century spread dramatically faster in far huger amounts than in OTL. The absence of Christian Byzantium, along with the Orthodox Church meant that Islam had no practically no difficulty growing in its range of influence. It ends up installing itself as the primary faith of Russia along with the rest of eastern Europe reaching as far as present day Germany. Western Rome faces its own demise In the 8th century AD, when the Islamic Moors, the rulers of North Africa and their fellow Muslims from the east forge an alliance to unleash a campaign on the southern and eastern fronts of the functioning Western Roman Empire. The Moors succeed in conquering western Europe, reaching both Iberia and France, defeating Charlemagne. It is only a matter of time before West Rome as a whole, Including the British Isles fall to Islamic rule. The Empire is renamed The Great Western Caliphate of the Moors by the conquerors. In the long term, the new languages are born as the result of the several local languages of the Empire mixing with Arabic.
Just as in OTL, the pagan Vikings from the northeast attack various regions throughout Europe, causing great damage in many areas, including the newly established Muslim Empire. However, due to the more unified military might of the Caliphate, the Vikings are much more easily defeated and barely manage to recover. Rather than ending up mostly Christian as a whole many Vikings convert to Islam, while another half manage to keep their faith for years to come. The Vikings themselves eventually face a civil war between the Muslims and those who remain pagan, which results in Islamic Vikings winning quickly due to support from the Muslim Russians from the east. By the end of the 11th century, Scandinavia, along with the entirety of Europe is of Muslim majority as a whole.
Renaissance and Colonization[]
The Crusades never occur in this ATL for obvious reasons and European Muslims manage to form strong ties with their eastern neighbors. The result of this, along with Islam's generally greater lenience towards advancement in science and technology than Christianity, the so-called Dark Ages ends much sooner. The Renaissance era happens centuries earlier in the 1100's (as opposed to the 1400's in OTL), causing Europe and its population to grow more rapidly in both enlightenment and in technological advancement, thriving to an even greater height than in the OTL Renaissance. The skills in metallurgy, pottery and textiles advance much faster along with chemistry, astronomy along with many other things the Arabs were masters at. Immense economic power lead to the Industrial Revolution occurring by the end of the 15th century.
The near invincible Alliance of Muslims of Europe, The Middle East and North Africa managed to easily repel the Mongol invaders hailing from the the East, thus paving the way for even more socio-political stability and the protection of scientific advancements and discoveries.
Just as this ATL's Industrial Revolution begins, The New World is discovered through sea faring as in OTL, with Mecca quickly informed. The Holy city agrees to have The New World divided into two different colonization zones. Nearly everything from Panama to Quebec is colonized by the Western Caliphate, whereas most of South America, starting with Brazil is colonized by The Ottoman Empire.
Unlike the initial European colonizers, the Spanish and Portuguese in OTL, The Ottomans and Moors were much more enlightened, secular and somewhat less xenophobic and did not view the native civilizations and cultures such as the Mayans, Aztecs and Incas in need of being completely destroyed and assimilated. Instead, they simply spread technology and faith throughout the Native empires, although several small-scale skirmishes did occur.
Many of The Aztecs in particular, became highly intrigued with Islam, since it did not require human sacrifice like their own original faith. Teaming with the Moors, the newly Muslim-converted Aztecs launched a Jihad against the Aztec Empire's ruling class which included the priests and the Emperor, until they finally submitted to Islam.
Eventually in the Moor Caliphate, despite the common faith of the empire, ethnic rivalries grew as the economies and colonies thrived. In the late 16th century, several proportions of the Caliphate gradually declared independence and formed rough approximations of current OTL countries, along historic ethnic and cultural lines starting with the British Isles. Shortly afterwards, the Moorish colonies throughout North America were quickly left on their own to struggle for decades to come following their mother country's fate.
The Ottomans took advantage of their neighboring Empire's dissolution and seized control of nearly all the trade routes surrounding Africa and colonizing a large proportion of the continent as well. The Islamic Aztec Empire, along with the former Moor colonies to the North act as allies towards periodic Imperial Ottoman aggression.
Modern Era[]
In the present day, 90% of the world's populace is Muslim. Islam permanently occupies the Indian subcontinent, Burma, Thailand, most of China, nearly all of Africa and eventually the Americas and Australia via colonization. The Pacific coast of Asia and Japan, The Southernmost tip of India along with Sri Lanka still remain strongholds of faiths other than Islam such as Hinduism, Sikhism, Buddhism, Shinto, etc.
The local New World civilizations and cultures survived, except with their populations becoming largely split between the indigenous and colonizing faiths.
Alcohol is far rarer and largely unknown. It is mostly confined to what little there is of non-Muslim territories.
Physical traits such as Blonde hair and Blue eyes are far rarer in this world due to the European population breeding with Muslims rather than Northern Europeans (e g. Vikings), who along with their genealogy remained confined to their native region.
This ATL's equivalent of the United States of America is The Holy United Emirate (HUE), which was formed by the union of several North American colonies after decades of strife and instability following the Moor Caliphate's breakup. Today, it consists of present day OTL New England, the Midwest, Florida, Central United States, Central Canada and the Oregon, Washington State, southern British Colombia, Quebec, Ontario and the Canadian Maritimes. The Aztec Caliphate includes most the OTL's Spanish colonial territories including Mexico, California, Nevada, Texas, New Mexico, among other places.