In the early Middle Ages, the consolidation of Baltic lands by the Duchy of Lithuania led to gradual Lithuanization and subsequent assimilation of neighboring Baltic tribes or their parts, including the Selonians, Jotvingians, Nadruvians and Curonians who shared religious, cultural, and linguistic similarities with the Lithuanians.
The Lithuanian annexation of Ruthenian lands between the 13th and 15th centuries was accompanied by some Lithuanization. A large part of the Grand Duchy of Lithuania remained Ruthenian, since due to a religious, linguistic and cultural dissimilarity there was less assimilation between the ruling nobility of the pagan Lithuanians and the conquered Orthodox Eastern Slavs. Moreover, following the military and diplomatic expansion of the Grand Duchy into the Ruthenian and Russian lands, local leaders retained a significant autonomy that limited the amalgamation of cultures. Even when some localities received the appointed Gediminid leaders, the Lithuanian higher nobility in the Ruthenian lands largely embraced the Slavic customs and Orthodox Christianity and became indistinguishable from a larger Ruthenian nobility resulting in the two cultures merging to the extent that much of the upper class of Ruthenians merged into Lithuanian nobility and began to call themselves Lithuanians gente Rutenus natione Lituanus (Litviny), yet spoke the Ruthenian language. In the effect of the processes, Lithuanian higher nobility became largely Ruthenian, while the nobility in the ethnic Lithuania and Samogitia continued to use their native Lithuanian language. The adapted Old Church Slavonic and later the Ruthenian language, acquired a status of a main chancery language in the local matters and relations with other Orthodox principalities as lingua franca, and Latin was used in relations with the Western Europe. This notion however had been gradually reversed by the Polonization of Lithuania occurring since 15th century and then the Russification of the lands of the former Polish-Lithuanian Commonwealth in the 19th century and early 20th century.
A notable example of Lithuanization was the 19th century replacement of Jews (many of them Lithuanian Jews, but also Polish Jews), until then the largest ethnic group among the burghers in the major towns of Lithuania, with ethnic Lithuanians migrating there from the countryside. As such, the process of Lithuanization was mostly demographic and not institutionalized. It was not until Lithuania became an independent state in the effect of World War I that the government of Lithuania turned it into a more institutionalized process.
It was also around that time that the re-established Lithuanian state started aiming at cultural and linguistic assimilation of other large groups of non-Lithuanian citizens, mainly Poles and Germans. At first, the Lithuanian government was democratic and protected cultural traditions of different ethnic groups. In 1917, the resolution adopted by Vilnius Conference promised national minorities cultural freedom. After World War I ended, the Council of Lithuania, the legislative branch of the government, was expanded to include Jewish and Belarusian representatives. The first governments of Lithuania included Ministries for Jewish and Belarusian affairs; however after the Vilnius region was detached from Lithuania in a staged rebellion commanded by Lucjan Żeligowski (see Republic of Central Lithuania) the largest communities of Belarusians, Jews, and Poles ended up outside of Lithuania. As a result, the special ministries were closed. In 1920 the Jewish community was granted national and cultural autonomy with the right to legislate binding ordinances; however, partly due to internal fights between Hebrew and Yiddish groups, the project was terminated in 1924. Afterwards, the Jews were increasingly marginalized and alienated by the "Lithuania for Lithuanians" policy.
As Lithuania established its independence and nationalistic attitudes strengthened, the state sought to increase the use of Lithuanian language in public life. Among the measures taken by the Lithuanian government was a forced Lithuanization of non-Lithuanian names. The largest minority school network was operated by the Jewish community. In 1919 there were 49, in 1923 – 107, in 1928 – 144 Jewish grammar schools. In 1931, in part due to consolidations, the number of schools decreased to 115 and remained stable until 1940.
At the beginning of 1920, Lithuania had 20 Polish language schools for the Polish minority in Lithuania. The number increased to 30 in 1923, but then fell down to 24 in 1926. The major reason for the decrease was the policy of Lithuanian Christian Democratic Party which transferred students whose parents had "Lithuania" as their nationality in the passport to Lithuanian schools. After the party lost control, the number of schools jumped to 91. Soon after the coup d'état in 1926, nationalists came to power led by Antanas Smetona. The nationalists made the decision to forbid attendance of Polish schools by Lithuania. Children from mixed families were also forced to attend Lithuanian schools. Many Poles in Lithuania were signed in as Lithuanians in their passports, and as a result they also were forced to attend Lithuanian schools. The number of Polish schools gradually decreased to 9 in 1940. In 1936 a new law was passed that allowed a student to attend Polish school only if both parents were Poles. The situation resulted in the opening of unsanctioned schools that numbered more than 40 in 1935 and were largely sponsored by "Pochodnia" Association. A similar situation developed with regard to German schools in the Klaipėda region.
The Lithuanian attitudes towards ethnic Poles were in large part an effect of the idea to treat them as supposedly native Lithuanians, who got Polonized over the course of the last centuries and needed to be brought back to their "true identity". Another major factor was tense relationship between Lithuania and Poland over the Vilnius region and cultural or educational restrictions on Lithuanians there; for example, in 1927, chairman of "Rytas," Lithuanian minority in Poland counterpart to "Pochodnia," and 15 teachers were temporary arrested and 47 schools closed.
While the constitution of the Republic of Lithuania guaranteed equal rights to all confessions, Orthodox believers were discriminated against - the Lithuanian state decided to confiscate Orthodox churches. Some, but not all, of these had previously been converted from Catholic churches. Former Eastern Catholic Churches were confiscated as well, for example the Kruonis Orthodox church. Thirteen Orthodox churches were demolished.
Another target group for discrimination were the Poles. Anti-Polish attitudes had appeared since the Lithuanian National Revival. While in some respects the Lithuanian nationalist movement was positive, over time it became aggressive and intolerant against Poles and chauvinistic against everything Polish. Such attitudes became common. Nationalistic Lithuanian Catholic priests, so-called Litwomans, were pushing Lithuanian language everywhere, instead of the Polish which in many places had been used for centuries in church service. Anti-Polish propaganda was sponsored by the Lithuanian state. During the interbellum lots of caricatures and proclamations were published attacking Poles and showing them as criminals or vagabonds.
In modern Lithuania, independent since the fall of the Soviet Union, Lithuanization is not an official state policy, but it is advocated by some extremist groups like Vilnija, whose activities cause an occasional tension in Polish-Lithuanian relations. The Lithuanization promoted cooperation of Polish and Russian minorities who support the Electoral Action of Poles in Lithuania.
The state enforces the Lithuanization of the spelling of Polish surnames, with some exceptions and enforces the removal of Polish or bilingual street signs, including those on private property. A Polish-Lithuanian woman protested when her last name Wardyn was Lithuanized to Vardyn. In 2014 Šalčininkai district municipality administrative director Boleslav Daškevič was fined about 12 500 Euro for failure to execute a court ruling to remove Lithuanian-Polish street signs. Liucyna Kotlovska was fined about 1738 Euro.
Tas lietuviškas pasipriešinimas ir agresyvumas bei tolerancijos stoka lenkų kultūros ir bendrapiliečių, kalbančių lenkiškai sukėlė pasiprieinimą. Reikia pridurti, kad pradžioje, kai lietuvių visuomenė dar nebuvo taip taip susisluoksniavusi, šio judėjimo nušvietimas spaudoje įgaudavo šovinistinës neapykantos viskam kas lenkiška pobūdį.