Mandić unconditionally supported the move, reasoning that the annexation would make it easier for the two regions to later be united with the nominally autonomous Kingdom of Croatia-Slavonia.

[1] Following the establishment of the Kingdom of Serbs, Croats and Slovenes, Mandić was chosen to become a deputy in the National Assembly, but never took office because prominent Serbian politicians vetoed his appointment.

[citation needed] In 1920, Mandić was appointed to the constituent assembly of the Kingdom of Serbs, Croats and Slovenes, representing the Croatian Popular Party (Hrvatska pučka stranka).

[1] Mandić was living as a retired government functionary at the time the Independent State of Croatia (Nezavisna Država Hrvatska, NDH) was declared.

His appointment was met with mixed feelings, and infuriated politicians such as Mladen Lorković, Mile Starčević and Vladimir Košak, some of whom threatened to resign due to the decision.

He advocated the creation of a coalition government, while prominent HSS member August Košutić voiced support for a clerical, partisan one which distanced the Ustaše from Croatian state politics.

[1] On 1 March 1944, Mandić and Croatian Foreign Minister Stijepo Perić visited Adolf Hitler at the Schloss Klessheim, a Baroque palace located 4 km (2.5 mi) west of Salzburg.

[6] Mandić and Perić complained to Hitler that the staff officers of the 13th Waffen Mountain Division of the SS Handschar (1st Croatian) were promoting the autonomy of Bosnia and Herzegovina.

"[7] In March 1945, Mandić called for NDH citizens of all ethnicities to voice their thoughts on the Ustaše, the war and the communist Yugoslav Partisans.

With his backing, the NDH drafted a memorandum to British Field Marshal Harold Alexander, Commander-in-Chief Middle East and commander of the 18th Army Group in Tunisia, expressing a desire to defect to the Allies in the wake of Hitler's death.