STORY: Speaking to reporters aboard a plane returning from a three-day trip to Kazakhstan, Francis also urged Kyiv to be open to eventual dialogue, even though it may "stink," because it would be difficult for the Ukrainian side.
The war in Ukraine, which Russia invaded on February 24, provided the backdrop to the pope's visit to Kazakhstan, where he attended a congress of religious leaders from around the world.
In a 45-minute-long airborne news conference, a reporter asked if it was morally right for countries to send weapons to Ukraine.
He expounded on the Roman Catholic Church's "Just War" principles, which allow for the proportional use of deadly weapons for self defence against an aggressor nation.
Explaining the difference between when it is moral or immoral to supply weapons to another country, Francis said:
"It can be immoral if the intention is provoking more war, or to sell arms or dump arms that (a country) no longer needs... Defending yourself is not only something licit, but it is also an expression of love for your country." he said.