Lithuanian raison d'état, also called Lithuanian particularism () and Lithuanian separatism (; ), was a major force throughout Lithuanian history with regard to Lithuanian and Polish relations, showing itself in Lithuanian efforts to maintain Lithuania's political, legal, cultural, and military distinctiveness from Poland from the Late Middle Ages to the interwar period. As the Polish historian Tomasz Ambroziak noted, the terms are not precisely defined, although the concept has been repeatedly addressed in academic works. The terms "Lithuanian separatism" and "Lithuanian particularism" have a long term interchangeable usage in historiography.
Throughout all of these centuries, the close relations did not mean the complete erasure of Lithuanian distinctiveness, but instead saw the Lithuanian raison d'état repeatedly reassert itself. Frequently, it was a defensive reaction to recurrent Polish attempts to completely incorporate Lithuania into Poland.
In the Grand Duchy of Lithuania The Grand Duchy of Lithuania had close relations with the Kingdom of Poland since 1385. Despite this, the Lithuanians frequently manoeuvred for more political room with regard to Poland by maintaining a separate office of the Grand Duke of Lithuania, the Sejm, the Lithuanian Council of Lords, and other institutions.
The Lithuanian nobles rejected Lithuanian Grand Duchess Uliana of Tver 1383 project to Christianize Lithuanians as Eastern Orthodox and instead Lithuanian monarch Jogaila, along with other Gediminids, on 14 August 1385 in the Kreva Castle agreed to conclude the Union of Krewo, according to which Jogaila married Polish monarch Jadwiga and was crowned as Polish King Władysław II, while the Lithuanian and Ruthenian territories of the Grand Duchy of Lithuania were attached () to the Crown of the Kingdom of Poland and Poland's suzerainty over Lithuania was set. The Union of Krewo laid the foundations for the Polish nobles to request termination of Lithuania's statehood and complete integration of Lithuania to Poland. Nevertheless, soon the Lithuanian Civil War (1389–1392), a sequel to the Lithuanian Civil War (1381–1384), occurred as a struggle for power in the Grand Duchy of Lithuania between the cousins Władysław II Jagiełło and Vytautas the Great, a defender of the statehood of the Grand Duchy of Lithuania which drawn support for him of a part of Lithuanian nobles. The 1381–1384 Lithuanian Civil War ceased with the Ostrów Agreement, signed on 4 August 1392, according to which Vytautas the Great regained control of the Duchy of Trakai, other lands of his father Kęstutis, and was granted the right to rule Vilnius, the capital city of the Grand Duchy of Lithuania, as regent and vassal of the King of Poland and Grand Duke of Lithuania Władysław II Jagiełło, however Vytautas the Great soon began consolidating his reign as Grand Duke of Lithuania, not as regent.
In the late 14th century a set of three acts were passed in Vilnius, Grand Duchy of Lithuania, and they were confirmed by the Polish Crown Council in Radom, Kingdom of Poland in 1401 (see: Pact of Vilnius and Radom), according to which the Kingdom of Poland legally confirmed the sovereignty of the Grand Duchy of Lithuania and recognized Vytautas the Great as the Grand Duke of Lithuania () until his death, but henceforth Władysław II Jagiełło held a higher monarchial title Supreme Duke of Lithuania ().
On 2 October 1413 in the Polish town Horodło the Union of Horodło was concluded where the first act was signed by Władysław II Jagiełło, King of Poland, and Vytautas the Great, Grand Duke of Lithuania, the second and third acts were composed by the Polish nobility (szlachta) and Lithuanian nobility boyars, respectively, which amended the earlier Polish–Lithuanian unions of Krewo and Vilnius–Radom, politically Lithuania received more autonomy as, after the death of Vytautas the Great, the Lithuanian nobles were permitted to choose another Grand Duke of Lithuania instead of passing this title to Władysław II Jagiełło or his heir, and prevented the Polish nobles aspirations to incorporate the Grand Duchy of Lithuania into the Kingdom of Poland. However, the Union of Horodło intensified the Polonisation of the Grand Duchy of Lithuania as it adopted the Polish institutions (castellans and voivodes) and forty-seven selected Lithuanian nobles were adopted by Polish families and granted Polish coats of arms. Moreover, the Lithuanian nobles promised to the Polish nobles to maintain the Polish–Lithuanian Union and after the death of Vytautas the Great to elect the new Grand Duke of Lithuania only with agreements of the King of Poland and Polish nobles, which meant the King of Poland and Polish nobles suzerainty over Lithuanian nobles and Grand Duchy of Lithuania, but Lithuanian nobles were permitted to participate in the royal elections in Poland.
The political and military efforts of Lithuanian Grand Duke Vytautas the Great and his supporters Lithuanian nobles secured a separate monarchial institution of the Grand Duke of Lithuania and separate management organization of the Grand Duchy of Lithuania. Since 6 January 1429 the 13-week period Congress of Lutsk was held in the Lubart's Castle in Lutsk, Grand Duchy of Lithuania where Vytautas the Great was offered to be coronated as the King of Lithuania and this way restore the Kingdom of Lithuania. The royal coronation of Vytautas the Great was suggested by Sigismund, Holy Roman Emperor, as a result of Lithuanian nobles diplomatic efforts following the 1398 Treaty of Salynas and initially during the Congress of Lutsk was also supported by Polish King and Supreme Lithuanian Duke Władysław II Jagiełło, however it was opposed by Polish nobles, Polish Royal Secretary Zbigniew Oleśnicki and eventually pressurred by them Władysław II Jagiełło cancelled his initial approval for the royal coronation of Vytautas the Great. Vytautas the Great sought the royal coronation as the King of Lithuania in the Vilnius Cathedral to strengthen the sovereignty of the Grand Duchy of Lithuania and to dismiss the Polish nobles aspirations to incorporate the Grand Duchy of Lithuania into the Crown of the Kingdom of Poland, however Vytautas the Great died on 27 October 1430 before his royal crown arrived to the Grand Duchy of Lithuania.
The Lithuanian nobles selected Švitrigaila as the new Grand Duke of Lithuania and he continued Vytautas the Great policies as he defended the sovereignty of the Grand Duchy of Lithuania and sought to be crowned as the King of Lithuania, however his poor relationships with the Polish nobles, Władysław II Jagiełło and lost two decisive battles (Battle of Ashmyany in 1432 and Battle of Wiłkomierz in 1435) resulted in his overthrow as the Grand Duke of Lithuania by Vytautas the Great brother Sigismund Kęstutaitis and his supporters. Sigismund Kęstutaitis initially supported Švitrigaila's struggle with Poland, but after the coup d'état against Švitrigaila (supported by Poland and a part of Lithuanians) and becoming the new Grand Duke of Lithuania he concluded the Union of Grodno (1432), according to which he ruled the Grand Duchy of Lithuania as regent of Władysław II Jagiełło and promised not to seek the Lithuanian royal crown for himself and his son Michael Žygimantaitis. Nevertheless, Sigismund Kęstutaitis in 1438–1439 sought to form an anti-Polish coalition with Albert II of Germany by seeking to annul his earlier agreements with Poland, but was assassinated in 1440.
In 1440, Casimir IV Jagiellon was sent by his older brother King of Poland and Hungary, Supreme Duke of Lithuania Władysław III to the Grand Duchy of Lithuania to rule in his name. But instead he was elected by the Lithuanian nobles as the Grand Duke of Lithuania upon his arrival to Vilnius on June 29, 1440, with the ringing of church bells and the singing of the Te Deum laudamus. This was breaching the agreements of the Union of Grodno (1432) and terminating the Polish–Lithuanian union. It manifested Lithuania as a sovereign state and its ruler Casimir IV Jagiellon stressed himself as a "free lord" (pan – dominus). According to historian Edvardas Gudavičius, Bishop of Vilnius put a Gediminas's Cap in the Vilnius Cathedral on his head, despite the Polish nobility's opposition. Following the death of Władysław III in the Battle of Varna (1444), Lithuanian Grand Duke Casimir IV Jagiellon, as requested by Lithuanian nobles, agreed to the Polish nobles offer to also become the King of Poland by only forming an alliance of two equal rights countries and concluding a personal union.
The Lithuania–Poland relations only grew closer when both states joined the Polish–Lithuanian Commonwealth in 1569. In 1563–1564, Lithuanian Grand Marshal Mikołaj Radziwiłł the Black, known for resolutely defending the sovereignty and territorial integrity of the Grand Duchy of Lithuania, led Lithuanian delegation which travelled to the Sejm of the Kingdom of Poland in Warsaw due to Lithuanian military failures in the Livonian War and accepted to conclude only a limited union of Lithuania and Poland without the unification of the two countries into one country, but it was not concluded and Mikołaj Radziwiłł the Black died on 28 May 1565. While further negotiating the closer Polish–Lithuanian Union during the Sejm held in Lublin since 10 January 1569 the highest Lithuanian delegation officials, including the head of delegation Lithuanian Grand Chancellor Mikołaj Radziwiłł the Red, Lithuanian Vice-Chancellor Eustachy Wołłowicz, Samogitian Elder Jan Hieronimowicz Chodkiewicz, Kievan Voivode Konstanty Wasyl Ostrogski, sought to maintain the statehood of the Grand Duchy of Lithuania, demanded for a separate Lithuanian parliament, separate proclamation in Vilnius of the Grand Duke of Lithuania and that joint Polish–Lithuanian parliament sessions be held alternately in Lithuania and Poland, while the Polish senators and members of parliament treated earlier Polish–Lithuanian Union acts as Lithuania's incorporation into Poland and demanded to conclude the Polish–Lithuanian Union as such. As monarch Sigismund II Augustus demonstrated support for the Polish nobles demands and transferred a part of the Grand Duchy of Lithuania territories to the Crown of the Kingdom of Poland (Kiev Voivodeship, Podolian Voivodeship, Bracław Voivodeship, Volhynian Voivodeship, Podlaskie Voivodeship), the head of the Lithuanian delegation Mikołaj Radziwiłł the Red refused to further participate in the Polish–Lithuanian Union negotiations and he was replaced by Jan Hieronimowicz Chodkiewicz as the head of the Lithuanian delegation. Since 6 June 1569 during further negotiations of the Polish–Lithuanian Union in Lublin, Polish nobles formally agreed that the Grand Duchy of Lithuania would be treated as equal with the Crown of the Kingdom of Poland and the Lithuanian delegation eventually agreed to a closer Polish–Lithuanian Union, which was adopted by monarch Sigismund II Augustus and, thanks to the diplomatic efforts of Chodkiewicz, preserved a separate Grand Duchy of Lithuania territory, monarchial title (Grand Duke of Lithuania), seal, administration, laws, courts, treasury, army. Nevertheless, Mikołaj Radziwiłł the Red refused to sign the 1569 Union of Lublin, viewed it as the funeral and eternal destruction of a free and independent state of the Grand Duchy of Lithuania, and following the death of monarch Sigismund II Augustus sought to decrease the consequences of the 1569 Union of Lublin. According to Lithuanian historian Saulius Sužiedėlis, the Lithuanians resistance to the Union of Lublin was an early modern times example of Lithuanians emphasis of a separate Lithuanian national identity and consciousness.