Why would religious justifications be necessary for wars in the 20th century? nations of the same religion were often at war with each other...

First Ethiopian war:
The church in 1861 deeply discouraged or even forbade catholics to take part in Italian politics; "Non expedit - Neither elector nor elected". This lasted at least until the 1905.
Anyway I have not found anything about any reaction of the Church to the war.

Second Ethiopian war: The pope strongly condemned the war, then Mussolini warned him that his words could destabilize relationship between the Italian government and the Vatican.

The pope's tone was much more moderate after that, and a few months later during the "Oro alla patria" campaign (the fascist government made a huge campaign asking people to donate gold to the regime to finance the war and boost nationalism) the archbishop of Milan even gave his divine blessing to the Italian army who was going to "bring catholic faith and Roman culture to Ethiopia".

Many catholic intellectuals and some people in the vatican openly disagreed with the Church's stance, but it didn't matter much. The vatican didn't even say anything about the toxic gasses used by the Italians in the war.