One day the Byzantine Empire was having a royal marriage with Moscowy, when suddenly, the Ottoman Empire declared war using the holy war casus belli!
'Muahaha' said the Ottoman Empire. 'I will annex you and move my capital to Thrace and there is nothing you can do about it because you only have two provinces!'
'Oh yeah?' the Byzantine Empire replied. Well suck my fleet, you upstart little Turkic cunt.' The Byzantine Empire hired an admiral and all of his stats were maxed out and he drove his single galley into the Ottoman fleet and captured every single one of the Ottoman ships. Then the Byzantine Empire converted Emperor Badassicus in to a general and steamrolled the Ottomans army even though it was 100000k strong.
'Arrrgh!' said the Ottoman Empire. 'Foiled again!' and the Byzantines annexed the Ottoman Empire even though it was far too large to do that because the Byzantines were just too cool.
'OMG' said Castile. 'You are so prestigious and wealthy and have a very low reputation! Civil union me! Civil union me hard!' and the Byzantines did but they were too awesome for Castile to handle so Castile's ruler's head exploded and the Byzantine Empire inherited.
'Fak!' said France. 'The Byzantine's are too strong and are now coterminous with me! England please save me because I am a little French bitch and cannot defend myself!'
England and France formed an alliance and stormed North Iberia with their mighty armies and the OP English Navy but the Byzantines were too awesome for that and they attacked the French armies with a single regiment of infantry and won because the infantry had 10000000% discipline because it was a badass Varangian hunk-o-man.
'Fak, Byzantium' said England, 'you are so strong we should align and kick some French arse.'
So Byzantium aligned with England and vassalised France and then personal unioned England's brains out all over Western Europe and shot his merchants all over England's CoT.