Northern Crusades: Danish victory at the Battle of Lindanise (modern-day Tallinn) establishes the Danish Duchy of Estonia.

The Northern Crusades or Baltic Crusades were Christianization campaigns undertaken by Catholic Christian military orders and kingdoms, primarily against the pagan Baltic, Finnic, and West Slavic peoples around the southern and eastern shores of the Baltic Sea.

The Battle of Lyndanisse or Lindanise was fought on 15 June 1219 during the Northern Crusades, between the forces of the invading Kingdom of Denmark and the local non-Christian Estonians. The Danish victory in the battle, at the site of the later Hanseatic city of Reval helped King Valdemar II of Denmark to subsequently claim the territory of northern Estonia as his participation in the crusade into Estonia had been undertaken in response to calls from the Pope.

Tallinn is the capital and most populous city of Estonia. Situated on a bay in north Estonia, on the shore of the Gulf of Finland of the Baltic Sea, it has a population of 456,518 as of 2025 and administratively lies in Harju County. Tallinn is the main governmental, financial, industrial, and cultural centre of Estonia. It is located 187 km (116 mi) northwest of the country's second largest city, Tartu, however, only 80 km (50 mi) south of Helsinki, Finland; it is also 320 km (200 mi) west of Saint Petersburg, Russia, 300 km (190 mi) north of Riga, Latvia, and 380 km (240 mi) east of Stockholm, Sweden. From the 13th century until the first half of the 20th century, Tallinn was known in most of the world by variants of its other historical name Reval.

The Duchy of Estonia, also known as Danish Estonia, was a direct dominion of the King of Denmark from 1219 until 1346 when it was sold to the Teutonic Order and became part of the Ordensstaat.