Prof. Karvaš participated in antifascist resistance in August 1943. He co operated with group of Dr. Vavro Šrobár till the second half of year 1944. He established contacts with members of illegal Slovak National Council through Dr. Ing. Zaťko and they informed him about preparations of the Slovak National Uprising. His task was to supply the uprising from the economic aspect. Prof. Karvaš used the bombing of Bratislava in June 1944 and ordered to transfer about 3 billion crowns from Slovak National bank in Bratislava to branch office of the Slovak national bank in Banská Bystrica. This money were available for the insurgents after the declaration of the Slovak National Uprising. In spite of conspiration Gestapo uncovered the relations between Karvaš and antifascists and arrested him in his own apartment on September 3, 1944. After interrogation on Gestapo in Bratislava he was interned in concentration camps of Flossenburg, Dachau, Innsbruck and Niederdorf.
After his comeback from the concentration camp in 1945 he taught and worked as an adviser of the Czechoslovak government for the reconstruction of Slovakia. Prof. I. Karvaš occupied a post of decanus of the Faculty of Law and he was a chairman of International executive board of the health resort.

He refused to co-operate with communist regime and the State court in Prague found him guilty in crimen leasae majestatis in 1949. He lost his property and went to prison for 2 years. He was released after 1 year with health problems.

Within the frame of Action “B” his family was evacuated from Bratislava in 1953. He was second time found guilty for crimen leasae majestatis and sent to prison for 17 years in 1958. He became amnesty after 2 years and he was rehabilitated by court in 1969.

He died on February 20, 1981.