When conflicts arose with Germany over the interpretation of the convention, Sidzikauskas negotiated with German officials and represented Lithuania at the League of Nations.

At the end of the war, he joined the Supreme Committee for the Liberation of Lithuania (VLIK) and in 1947 became chairman of its Executive Council which had aspirations of becoming the Lithuanian government-in-exile.

[7] In spring 1918, authorized by the Council of Lithuania, Sidzikauskas organized the evacuation of Lithuanian students and teachers back to Vilnius.

[6] In March, Sidzikauskas, as a representative of the Democratic National Freedom League, was a member of a six-men commission that negotiated the formation of the coalition Cabinet of Prime Minister Mykolas Sleževičius.

[6] On 24 July, he personally traveled to Daugailiai on the Lithuanian–Soviet front to exchange 15 prominent Lithuanians, including Mečislovas Reinys, Liudas Gira, Felicija Bortkevičienė, Antanas Tumėnas, for 35 communists.

Sidzikauskas became heavily involved with negotiating Klaipėda's status and maintaining good relations with Germany which was the largest trading partner of Lithuania and its key supporter against Poland.

[11] Sidzikauskas supported the December 1926 coup by Lithuanian military and helped to convince President Kazys Grinius to resign in favor of Antanas Smetona.

[23] In April 1921, Sidzikauskas became a member of the Lithuanian delegation, led by Ernestas Galvanauskas, to negotiate with the Second Polish Republic regarding Vilnius Region which Poland captured as a result of the Żeligowski's Mutiny in October 1920.

[24] The negotiation was mediated by Paul Hymans whose plan was to transfer Vilnius to Lithuania if it agreed to join a federation with Poland.

[27] The Lithuanian delegation, led by Sidzikauskas, argued that, according to the Klaipėda Convention, the parliament had no rights to submit the complain (it could be done only by a member of the Council of the League of Nations).

[31] He could have argued that the right to petition the League of Nations was given to local inhabitants only in Article 11 of the Convention dealing with the protection of ethnic minorities.

This time they complained about Lithuania's interference in the local elections, such as press censorship, restrictions on freedom of assembly, promoting pro-Lithuanian bias within the electoral commission.

Lithuanians relented to many German demands, including appointment of all-German Directorate of the Klaipėda Region and less interference in the October 1930 local elections.

[34] German Minister of Foreign affairs Julius Curtius kept pressuring Lithuanians to recall Klaipėda governor Antanas Merkys.

[35] Sidzikauskas failed to resolve the conflict via direct negotiations and the complaint was discussed in the January 1931 session of the League of Nations.

[39] When further negotiations between Poland and Lithuania collapsed, the League of Nations referred the case to the Permanent Court of International Justice in January 1931.

[48] In October 1934, Kaunas district court found Sidzikauskas guilty and sentenced him to six months in prison that was replaced by a three-year parole.

[46] Despite the acquittal and Sidzikauskas' claims that it was a German provocation and revenge for his work in defending Lithuania's interests in Klaipėda, his reputation was ruined.

[46] Sidzikauskas completed his law studies at the Vytautas Magnus University in 1935 and became director of the Lithuanian subsidiary of the Shell Oil Company in 1936.

He contributed articles to Teisininkų žinios, Naujoji Romuva, Lietuvos ūkis, and was editor of Revue Baltique that was published jointly with Latvians and Estonians.

He became chairman of the Union of Lithuanian West (Lietuvos vakarų sąjunga) which sought to strengthen cultural ties between the Klaipėda Region and Lithuania.

[6] Sidzikauskas worked at an administrative office and managed to get his old diplomatic contacts to obtain a release from Heinrich Himmler in December 1942, after 20 months of imprisonment.

[52] Towards the end of the war, in October 1944, Sidzikauskas joined the Supreme Committee for the Liberation of Lithuania (VLIK) and became chairman of its foreign branch.

[54] VLIK asserted that the Diplomatic Service was its subordinate in its April 1945 memorandum to American General Dwight D. Eisenhower that was co-authored by Sidzikauskas.

[56] The parties met in July 1946 in Bern and agreed to form an Executive Council under VLIK, an embryo which they hoped could develop into a recognized government-in-exile.

[67] In early 1949, Sidzikauskas and Mykolas Krupavičius, chairman of VLIK, toured Lithuanian communities in the United States for more than three months.

[68] In August 1949, Sidzikauskas became chairman of the Lithuanian National Council (Lietuvos tautinė taryba) which joined the European Movement International in January 1950.

[80] ACEN and the Committee for a Free Lithuania wrote numerous memorandums and protest notes to world leaders reminding of the Soviet occupation.

For example, in May 1955, on the 37th anniversary of the Constituent Assembly of Lithuania, 21 former members of the assembly issued a memorandum to parliament members of United States, Canada, and Great Britain;[81] representatives of the Baltic states issued a manifesto on the 20th anniversary of the Soviet occupation in June 1960 (in response, the Parliamentary Assembly of the Council of Europe adopted a resolution condemning the occupation in September 1960);[82] ACEN held a meeting with the International League for the Rights of Man which issued a report condemning the Soviet regime as it violated human rights in 1962.

[84] All of these actions kept the issue of Lithuania's independence on the political agenda despite the general fatigue and increasing acceptance of the status quo in Eastern Europe.