And because he's been excommunicated there's certain places he can't go, so they have to make—take, like, the most dangerous, most steep route across the Alps.
This was a smidge problematic, Crusading-wise, because Zara was a Christian city, but the Crusaders agreed to help, resulting in the Pope excommunicating both them and the Venetians.
Mary's position as a Catholic heir threatened Queen Elizabeth I's precarious position as Queen of England. England had been excommunicated by the Pope, inviting Catholics to dethrone her.
But that, many decided, was old enough to lead the faithful to the promised land — even though he had officially been excommunicated from the Mormon church before Smith died.
Louis saw this request as a win-win-win, help a friend, please his wife, and mess over a rival, except it led to the Pope excommunicating the new couple and Louis going to war with Champagne.