"Fulfilling military duty does not contradict religious beliefs," – the head of the Conference of the RCC in Ukraine
This is discussed in the article by Glavkom.
The fulfillment of military duty, not only in a state of war (mobilization), does not contradict the religious beliefs and doctrinal teachings of the Catholic Church. Every person has the right to defend their right to life. Therefore, by defending their life, we do not become guilty of murder, even if we are forced to kill an attacker (Catechism of the Catholic Church, para. 2264). Therefore, killing an enemy while defending one's country is not a sin. The Conference of Roman Catholic Bishops of Ukraine openly refers to the Russian Federation as an aggressor.
By the way, the Ukrainian Orthodox Church also does not consider killing occupiers as a mortal sin. "The destruction of Russian attackers by defenders of Ukraine who commit armed aggression against our people and try to seize the land given to us by God cannot be equated with the sin of criminal murder. This is a forced, just and sacrificial defense based on love for one's family, people and Homeland," the comment of the Ukrainian Orthodox Church states.