Pope Leo decried the use of military force as a means of achieving diplomatic goals, in an unusually sharp annual foreign policy speech on Friday in which he also called for the safeguarding of human rights in Venezuela.
Leo, the first U.S. pope, said the weakness of international organizations in the face of global conflicts was "a particular cause for concern".
"A diplomacy that promotes dialogue and seeks consensus among all parties is being replaced by a diplomacy based on force," Leo told some 184 ambassadors accredited to the Vatican.
"War is back in vogue and a zeal for war is spreading," said Leo, who was elected pope in May.
Referring to U.S. President Donald Trump's toppling last weekend of Venezuelan president Nicolas Maduro, the pope called for world governments to "respect the will" of the Venezuelan people going forward.
He said nations must "safeguard the human and civil rights" of Venezuelans.