Battle of Ankara
The Ottoman defeat by Timur in the Battle of Ankara of 1402 provided good conditions for an uprising, and the Christian forces crossed the Danube from the north into the Bulgarian lands: Mircea I, accompanied by Konstantin, invaded Dobruja, while Fruzhin together with the Serbian-supported Hungarian army entered the northwestern Bulgarian lands (around Vidin). These initially successful invasions were followed by an uprising of the Bulgarian population in some areas.