With a POD of no earlier than the death of Constantine VIII, preserve Byzantine control over Anatolia and (at least part of) S. Italy. If possible, avoid the Battle of Manzikert altogether.
With a POD of no earlier than the death of Constantine VIII, preserve Byzantine control over Anatolia and (at least part of) S. Italy. If possible, avoid the Battle of Manzikert altogether.
Alp Arslan actually wanted peace with the Byzantines, but Romanus was set on recapturing some border fortresses and decided to go to war anyway. So, have Romanus decide that those fortresses are too far from the imperial centre of power to reasonably hold (which is an assessment several modern historians have made), and agree to make (or, more accurately, keep) peace with the Seljuks instead. Hey presto, no Battle of Manzikert. And, whilst the Byzantine Empire and army would still face the same problems that made the Manzikert campaign such a disaster IOTL, they'd now be in a much better position to solve them.
This would be too overpowered, but we need a POD to to take DamascusUnder an effective ruler like Isaac Komnenos, or any good military leader even if he's a bad administrator, the Byzantines temporarily capture Damascus, taking prisoner the artisans and blacksmiths of its steel industry, resettling the prisoners of war in Constantinople, where they are freed and establish a manufactory with crucibles producing high-carbon Damascus steel, derived from Wootz steel, the same steel that started industrial metallurgy in Britain in the 18th century. After the loss at Manzikert worse than OTL, and a bloodier reconquest of the coasts of Anatolia (possibly a Pyrrhic reconquest of the interior which is then subsequently taken again by the Seljuks), there is a shortage of labor and excess of raw materials in the Byzantine Empire. The shortage of labor provokes investment by landholders into high-productivity agricultural equipment, and investment by the imperial state into high-productivity smelting and forging of weapons and armor.
Ultimately the growth in productivity leads to urbanization that recovers beyond even the high point of Basil II's reign (which was the highest GDP per capita realm of the entire Middle Ages), causing the proliferation of "cottage industries" manufacturing textiles and exporting them across Europe and the Mediterranean. These cottage industries based traditionally on waterpower, begin to use sources such as charcoal as well. Growing efficiency over time turns the Byzantine Empire into a North-Italian-style manufacturing center, importing raw materials from across the Mediterranean as well as producing raw materials of its own, and using those inputs to output manufactured products. The discovery of coal in Bulgaria leads to it being used as a power source for crucibles as well as textile manufactories. Using the airblowing mechanism on the crucible and existing steampowered contraptions known in the Byzantine empire as inspiration, a burgeoning Roman industrialist invents a coal-powered, so-called "steam engine", which increases efficiency of industrial enterprises beyond what mere water power can allow.
Voyages across the Mediterranean selling manufactured goods or buying raw materials are dangerous due to pirates and the turbulence of the sea, leading to the formation of joint-stock voyages, which then become joint-stock companies, perhaps influenced by Italian traders. Also inspired by Italian cities, the Byzantines establish a patent system that lasts 5-10 years, protecting innovators' rights to sell their inventions, rewarding innovation, before allowing unrestricted public use. Joint-stock companies officially invest in the production and sale of agricultural tools, textile products, ever-advancing clockwork machinery based on that already existed in the Empire, and the like. The Byzantine Empire starts to undergo a serious Industrial Revolution, giving it advances in productivity dwarfing the rest of the Mediterranean, allowing them to replenish weapons and armor, and finance wars, more effectively than any other state. Continued Komnenian rule as well as delegation to an expanding court ministry leads to stability, and the slow reconquest of parts of Anatolia as well as expansion into Serbia, the retention of Bulgaria, and expansion into Romania.
Increasing efficiency of Byzantine metallurgy also leads to cheaper production of iron. A Byzantine owner of an iron foundry, decides to build a two-tracked road interlaced with wooden ties between his plant and the mines that it sources from. This new "railroad" attracts the attention of several other companies, such as coal mining companies. The spreading network is noticed by the Emperor, who subsidizes the new railroad enterprises with state funding, leading to a Byzantium-wide railroad network. Faster transportation increases the industrial output of the Byzantine Empire as well as military's responsiveness to outside threats. While the new technologies filter out across the Mediterranean due to trade and investment, the Byzantines retain a distinct advantage during the Industrial Revolution of the 14th Century, similar to Britain in our 19th century. As the Byzantines expand northward into Eastern Europe, westward into Italy, and southward into the Middle East, acquiring coal, oil, and steel resources as well as vital farmland, fueling colonial expansion across the Indian and Atlantic Oceans, Byzantium becomes the largest empire the world has ever seen.