The Northern Crusades represent a period of history that was rather decisive and divisive for the Baltic Finnish tribe. A deep wedge was driven between the Finnic peoples' and that was the divide between Eastern and Western Christianity. The period is also historically interesting because with the increased interest of literate Europe directed towards the North, we finally start to get plentiful written accounts of our late iron age forefathers. Reason enough to start a thread on the topic.

Let's start with Wiki.

The Northern Crusades[1] or Baltic Crusades[2] were crusades undertaken by the Christian kings of Denmark and Sweden, the German Livonian and Teutonic military orders, and their allies against the pagan peoples of Northern Europe around the southern and eastern shores of the Baltic Sea. Swedish and German Catholic campaigns against Russian Eastern Orthodox Christians are also sometimes considered part of the Northern Crusades.[1][3] Some of these wars were called crusades during the Middle Ages, but others, including most of the Swedish ones, were first dubbed crusades by 19th century romantic nationalist historians. The east Baltic world was transformed by military conquest: first the Livs, Latgallians and Estonians, then the Semigallians, Curonians, Prussians and the Finns underwent defeat, baptism, military occupation and sometimes extermination by groups of Danes, Germans and Swedes.[4]

The official starting point for the Northern Crusades was Pope Celestine III's call in 1193; but the already Christian kingdoms of Scandinavia and the Holy Roman Empire had started to move to subjugate their pagan neighbors even earlier. The non-Christian people who were objects of the campaigns at various dates included:

-the Polabian Wends, Sorbs, and Obotrites between the Elbe and Oder rivers ( by the Saxons, Danes, and Poles, beginning with the Wendish Crusade in 1147 )
-the peoples of (present-day) Finland in 1154 (Finland Proper; disputed), 1249? (Tavastia) and 1293 (Karelia) (Swedish Crusades, although Christianization had started earlier),
-Livonians, Latgallians, Selonians, and Estonians (by the Germans and Danes, 1193–1227),
-Semigallians and Curonians (1219–1290),
-Old Prussians,
-Lithuanians and Samogitians (by the Germans, unsuccessfully, 1236–1316).

Armed conflict between the Baltic Finns, Balts and Slavs who dwelt by the Baltic shores and their Saxon and Danish neighbors to the north and south had been common for several centuries prior to the crusade. The previous battles had largely been caused by attempts to destroy castles and sea trade routes and gain economic advantage in the region, and the crusade basically continued this pattern of conflict, albeit now inspired and prescribed by the Pope and undertaken by Papal knights and armed monks.
I've read this book by Eric Christiansen, which is a good general kind of overview, but I find it somewhat lacking in local expertise and detail.



Here is what the region is thought to have looked like ethnographically at the time.



A map on the crusades:





The most interesting contemporary source is without a doubt the Chronicle of Henry of Livonia.