AHQ Without a 4th Crusade what happens to Bulgaria?

[legumes]

Like it says, if the 4th crusade or at least the 1204 sack of Constantinople and the subsequent partitioning of the East Roman Empire never occurred, what would happen with Bulgaria and the Balkans as a whole? Bulgaria had recently broken away from Byzantine control, declaring its second empire in the late 12th century. After the sack of Constantinople and the creation of the Latin Empire, they and the Crusader state waged a decades-long and bloody war with one another, with Bulgaria ultimately victorious, though they never seized Constantinople for their own.
If the Byzantines were still in on a mostly cohesive empire during the 13th century, how would they face the Bulgarian challenge, along with the later Serbians and other Slavic states? They'll certainly have more manpower than the Latins did, but their attention will be divided by having to deal with the Seljuks in Anatolia, and then the Mongols come storming in.
Fire away.

 

[Silver01]

well the mongols will smash bulgaria in 1241 around the same time they vassalize the seljuks

 

[Italian Sock Monkey]

What is the POD that prevents the sack of Constantinople? Do the crusaders pass through Byzantine territory peacefully? Do they pass through at all? Does Alexios III die before he can ask the Crusaders for help in restoring him to the throne? All of those would have an impact on the state that the ERE is in after a 'peaceful' 4th Crusade.

I doubt a reconquest of Bulgaria is possible. It took Basil II decades of consistent surgical pressure to completely subjugate Bulgaria. Which was only possible because the East was quiet and his economy was booming. The ERE of the 12th century was a ghost. It never recovered from Manzikert. The brief respite that Alexios and John II managed was eridicated by the utter misrule of Manuel and the civil wars/coups of the Angelos. I'd say Bulgaria is independent till at least the Mongol's arrive. What does Bulgaria do with it's independence? While the Empire was certainly squeezed from both East and West I don't think Bulgaria has the manpower for a full conquest of the Empire's balkan territories. Attacking Constantinople would certainly be suicide as well. The borders would be in constant flux which does not bode well for the ERE.

The Mongols will almost certainly crush Bulgaria especially if they've been at war with the ERE. How will the Khan respond to a diminished but united ERE? Will the ERE become an ally/enforcer of theres like Muscovy did? Will the Mongols decide to plunder the City for it's great wealth (assuming the Angelos don't destroy the treasury before the Mongols arrive.) Or will they just demand exorbitant tribute? It's difficult to say. I can see some sort of perfect cascade of events wherein the Mongols make the ERE their enforces and they take over management of Bulgaria and the Anatolian tributes payments. Then pull a Muscovy and incorporate all of them after the Mongols become too weak to challenge them. That however would take a competent Emperor and the Angelos never produced a single one so...