The Byzantine Empire, which lasted from May 11th, 330 CE to 1453 CE, was one of the most influential empires of history, with it having stopped the Umayyad Caliphate from invading the Balkans, and being the last legitimate claim to being the Roman Empire. It also was to an extent, its own downfall, as a series of extremely weak emperors caused them to lose HUGE territories in Anatolia and Europe.
However, you may wonder how it exactly fell, such as the details, and I can explain that, so get ready to read like 600 words.
Well, my innocent child, let me explain this to you, so during the Umayyad Caliphate's rise to power, they conquered MANY territories across the Mediterranean, and they would naturally try to attack the Empire that held so much territory right? Well they absolutely decimated the Byzantine Army during several battles, such as in The Battle of Ajnadayn, the first major battle between the two, taking place in modern day Syria, the forces that were put against each other was about ~60,000 Byzantines and ~20,000 Muslim forces, and the casualties were completely one sided, with the Byzantines losing around ~50,000 soldiers and the Muslims ~575 soldiers, yes, 575 to 50,000 casualties (From an Arab source from the 1200's, however modern estimates are unknown so take it with a grain of salt), absolutely devastating. And it is assumed that they lost because they had been taken by surprise, and were very ill-equipped, however you may be thinking “Hmm, but didn’t the Byzantines have great equipment?” and yes, they did, however these soldiers were also extremely terribly trained, so they were given very outdated equipment.
So the Muslim force, which consisted of 2 divisions (From Medina and Baghdad respectively) moved through the flat areas in Eastern Syria, where the Byzantine force was overwhelmed by the sheer speed, and rapidly retreated, leaving nearly 50,000 troops behind. So, they had continued losing in modern day Israel and Palestine, leading to their Egyptian territories being lost, and they retreated back to Lebanon and Syria. However, they didn’t hold on to these territories, at least reliably, as they continued taking huge casualties until their eventual retreat into Anatolia, where they were far more established.
However, this did not save them from their eventual downfall, as although they eventually had helped the other Christian states of Europe destroy the Caliphate, they had permanent scars in their populace from the insane losses, with it overall throughout the wars with them being about ~3,000,000 (40% of which were civilians) and they continued losing land to minor Anatolian states and in the Balkans & Italy. So, with the rising power of the Ottomans through diplomacy and war, the Byzantine’s would have several huge wars with them, which diminished their power even more, until, eventually on May 29th, 1453, Ottoman forces would breach the walls of Constantinople, putting an end to the aging empire.