Latgalians (Latin: Lethi, Letthigalli, Low German: Letti, Lethi, modern Latvian: latgaļi, letgaļi, leti; variant translations also include Latgallians, Lettigalls or Lettigallians) were an ancient Baltic tribe.
They likely spoke a variant of Latvian language, which probably became the lingua franca in present-day Latvia during the Northern Crusades due to their alliance with the crusaders. Latgalians later integrated with the neighbouring tribes, forming the core of modern Latvians.
The Lithuanian Crusade was a series of campaigns by the Teutonic Order and the Livonian Order under the pretext of forcibly Christianizing the pagan Grand Duchy of Lithuania. The Livonian Order occupied Riga in 1202 and in the 1230s they settled in Chełmno Land, a fief of Poland. They first conquered other neighboring Baltic tribes— proto-Latvians (Curonians, Semigallians, Latgalians, Selonians) and Old Prussians—in the Livonian Crusade and Prussian Crusade.
The first raid against the Lithuanians and Samogitians was in 1208. From then on, the orders played a key role in Lithuanian politics, but they were not a direct threat until the 1280s. By that time, the Grand Duchy of Lithuania was already a centralized state and could mount defenses. For the next century, the order organized annual colonialist reise (raids) into Samogitian and Lithuanian lands, without great success but at immense human cost. Border regions in Samogitia and Suvalkija became sparsely inhabited wilderness due to ethnic cleansing, although the order gained very little territory. The resulting wars between the Teutonic Order and Lithuania were one of the longest conflicts in the history of Europe.