He could call on the princes again, but I’m afraid they’re no longer there.
The Pope lost control of the Crusades in the Middle Ages, the Fourth Crusade being a major example.
Even so, who will answer the call? Are Protestants going to recognize the Pope’s authority to do such?
Are Catholics going to risk breaking the law of their states by organizing in a military manner outside their control?
Things like the Inquisition and Crusades are defensive tools that went out the window when Christendom was abolished in favor of secular nation-states.