[1][2][3][4] In September 1939, at the beginning of the Invasion of Poland and World War II, Musteikis supported Lithuania's neutrality and was against Adolf Hitler's call to Lithuanians to attack Vilnius.

After receiving the 1940 Soviet ultimatum to Lithuania on the night of June 14, 1940, during the last meeting of the Lithuanian government held in Kaunas, he urged that the ultimatum be rejected and argued for military resistance.

On June 15, 1940, he called on the 9th Infantry Regiment of Marijampolė to cover the departure of President Antanas Smetona and to symbolically resist the Army of the Soviet Union; the regiment marched but was stopped in Vilkaviškis.

[5][6] He then relocated to the United States in 1949,[7] where he lived in Roslindale[7] and then Dorchester, Massachusetts.

Musteikis was a recipient of the Order of the Lithuanian Grand Duke Gediminas 3rd Class (1929), the Order of Vytautas the Great 3rd Class (1935), and other medals.