Peter, according to what is probably a legend, had planned to join on the Crusade of 1101 and make a pilgrimage to Jerusalem at the urging of the monks of San Juan de la Peña, but Pope Paschal II had refused to allow it and ordered him to make war on Zaragoza instead.

Probably aided by knights from France and Catalonia, Peter certainly does make war on Zaragoza in 1101, in a campaign that lasts the whole year.

He may have been inspired by the First Crusaders, since contemporary accounts of the 1101 campaign call him a "cross-bearer" (crucifer).

The size of his forces so impresses a contemporary scribe in León that he remarks in the dating formula of a document of February 12 that "Peter, Aragonese king, with his infinite multitude of armed men, the city of Zaragoza, with Christ's banner, fought".

Peter has by June begun the siege of Zaragoza itself.

For the siege, he has a fortress built named Juslibol (a corruption of the Latin slogan Deus lo volt [”God wills it”] used by the First Crusaders) and rings the city with banners bearing the cross.

In August, he is conducting a razzia (raid) as far south as Alpenes and the river Ebro, but the campaign is eventually aborted due to insufficient cavalry.

By the end of the year he has expanded Aragon and Navarre in the west almost as far as the walls of Zaragoza and …