Migration Period
Thousands of people travel through europe due to Huns driving them out of eastern europe and asia. They eventually end up in western Rome and start to settle. This will have no consequences at all.
Crusades
The Byzantines have lost a major part of their territory to the invading seljuk turks. The Empire experienced a civil war which prevented it to defend properly. However, this came to an end and the new Emperor wants his lands back. He messages the pope for troops from the west.
Fall of Constantinopel
The Ottomans conquered Constantinopel, one of the big christian cities. It marks the end of the Byzantine Empire, and in some way, the Roman Empire. This is a shock to the christian world and surely will bring its consequences.
Reformation
The catholic church will split. This is what the pope fears, due to the multiple reformers like Martin Luther, John Calvin and Henry VIII, questioning the popes authority and the catholics church's ability to define Christian practice.
Napoleonic Wars
The 1st and 2nd Coalitions were won by France and saw Napoleon rise the ranks and eventually take over. However, the other european powers, still want to restore the old system and it may be, that there a more coalitions to come.
WW1
Years of imperialism and expansionist ambitions lead the european powers to heavily distrust each other, but their closest allies. No one knows, when the war will begin, but it's sure, that it can happen anytime.
WW2
The Great War should have been the war to end all wars. However, it looks like it only paved the way for another World War. Tensions increase, as fascist movements rise in europe and its countries.