During World War I as head of the Ukrainian parliamentary delegation, he struggled against the plans of Galicia annexation by Poland and was an adherent of the territory autonomy within Austria.

Being the head of the Galician delegation in Brest-Litovsk in February 1918, he favoured the introduction of Austria’s liability of granting autonomy to Galicia into the secret appendix of the International conference resolution.

Jointly with Czech and Slovakian parliamentarians he worked out a project about the formation of national states united with Austria on the Empires lands and submitted it for the Kaiser's consideration.

But in connection with the threat on the part of Poles, Lviv delegation of the National Council headed by Kost Levytsky seized power under arms and on 1 November proclaimed the formation of the West Ukrainian People's Republic (WUPR).

Being the president of the National Council, Petrushevych mainly performed representative functions, but owing to his political culture and parliamentary experience he had influence on making the most important decisions in the state.

The National Council elaborated a number of necessary laws, which regulated the socio-political and economical life, laid the legal foundation of the state and guarded it from acute social conflicts and destructive manifestations.

The president sent Ukrainian delegations to international talks in Riga and Geneva, sent notes and messages in the address of the League of Nations, got a resolution, which recommended a Council of Ambassadors in Paris to review the Galician problem.

[3] In spite of the emigrant's life difficulties, in old age he collaborated with the Ukrainian National Association, maintained relations with Hetman Pavlo Skoropadsky.