The article first appeared on the Today in Alternate History blog. The
scenario of a Byzantine Empire surviving until the Great War is fully
explored in Alexander Rooksmoor's latest AH novel Byzantine Express.
5 August, 1914 - Byzantine Empire Joins the Great War
The
clash of rival Empires known to alternate history as the Great War
rapidly escalated when Byzantium opened hostilities on the Central
Powers.
The imperial government in Constantinople recognized that
its survival over the centuries had depended upon the lasting support
of her long-time fighting partners, Serbia and Bulgaria. Set against
German encroachment in the Balkans, she unexpectedly found herself
allied with the British, French and Russians. These three rival Empires
were "fair-weather friends", having their own competing interests and
territorial ambitions in the near East. 
In fact, their only common interest was the ancient proverb "the enemy of my enemy is my friend".
Challenged
by such a powerful array of forces, the Central Powers were eventually
subdued, but Europe was shattered years by the unrelenting slaughter. In
the bloody aftermath of popular uprisings and continuation wars fought,
stateless minorities won their freedom and crowned heads were forced to
abdicate. Riding this sea change was the nascent Byzantine Republic. She
seemed incredibly fortunate to enjoy the unity of a Greek-speaking
population spread across a strategic territory on world trade routes,
Anatolia and the southern tip of the Balkans. In the early years of the
1920s she rapidly became a modern state at the forefront of efforts to
rebuild a broken continent.
The discovery of huge oil reserves in
the Levant changed everything. With the prospect of regional hegemony
returning unexpectedly into sight, the victor powers quickly became
deadly enemies. A group of right-wing officers known as the "Young
Byzantines" seized power in Constantinople. Convinced that the former
Imperium had fought on the wrong side of the Great War, they formed a
Fascist State and quickly set about occupying large swathes of Arabia.
Of
course, their encroachment into the Middle East was a cynical mirror
image of the failed earlier German land-grab on the Byzantine's own
door-step. With the Great Powers seemingly on the road to war for the
second time in a generation, it appeared that the ephemeral vision of
popular democracy that had first begun in Greece was a mirage. With the
world's oil supplies firmly in the greedy hands of the Young Byzantines,
W. B. Yeats bitterly noted that democracy was only a fleeting interlude
between lasting eras of demagoguery.
Author's Note:
In reality, Byzantium was conquered by the Ottoman Empire in AD 1453.
Provine's Pondering
As noted on Today in Alternate History, the timeline follows from a hypothetical, " Alexander Rooksmoor goes into deep reflection on potential changes in his Tablets of Lead blog post. In summary of the fascinating counter-factual analysis, the call for aid to crusaders from Western Europe allowed the Ottomans to recover their lands (rather than those lands being set up as Crusader States). Presumably, the big change was the Fourth Crusade where, as one History professor summed, "drunken Normans stormed Constantinople." Returned to power with a strong eastern buffer, the Byzantines withstand any incursions by Seljuk Turks. The later Ottomans (if Osman I isn't butterflied away from being born) would be one of several diverse states throughout the Muslim world farther southeast.
While digesting all this, multiple other points-of-departure may bubble up. One potential point-of-departure for such a TL could be even farther back with the incursion of the Seljuk Turks that began the call for Crusades. If the Battle of Manzikert of 1071 had been a rousing Byzantine victory rather than many of their mercenaries joining the Seljuk side, Byzantium could have maintained Anatolia and perhaps had to battle Mongols on their eastern frontier.
The extensive history of the Turkish people has plenty of PODs, including if they had never been converted to Islam and instead remained worshipers of Tengri. The Turks originated in northeastern Asia and migrated southwestward, where they came into the Muslim world via missionaries in Central Asia. Mercenaries and more formal armies made up much of the Seljuk push westward, moving into the territory conquered. If culture wars (and more literal wars) had broken out between the Turks and the Umayyad and Abbasid caliphates, that would had discouraged further migration.
On the Byzantine side, more alternatives come to mind. Perhaps if the Byzantines and Sasanids had not fought in the seventh century, conducting the final "Roman-Persian" war, there would not have been so much back-and-forth destabilizing the area. Or, what if the Plague of Justinian hadn't ravaged the Mediterranean economy and Justinian's conquests had time to affirm Byzantine rule and recoop wartime investments?
