Hence the only course open to Florence and the Tuscan communes in general was to seize every convenient occasion of asserting their rights either by craft or by violence. The war between Pisa and Lucca had already broken out, and as Count Guido, the foe of the Florentines, had joined with Lucca, they formed an alliance with Pisa. This treaty was very advantageous to their commerce, but it pledged them to an active share in the war.[159] They willingly undertook this, for it was an opportunity of fighting not only the Lucchese, but also the latter's patrons, Count Guido and Christian of Mayence. At first it seemed as though Pisa would be forced to make peace, for on March 23, 1173, Christian declared that city to be under ban of the Empire, thus stripping it of all the privileges it had previously enjoyed. In fact, on the 23rd of May an agreement was concluded (witnessed also by the Florentines) to the effect that Pisa and Lucca should proceed to an exchange of prisoners. The ban was raised on the 28th of the same month, and peace was solemnly proclaimed in Pisa on the 1st of June. But two months afterwards an unexpected event caused the war to be speedily renewed. The archbishop had invited the Consuls of Pisa and Florence to come to San Genesio on the 4th of August, and on their arrival had them promptly seized and cast into prison. What could have caused an act rendering war unavoidable, after such strenuous efforts to establish peace? Many explanations have been suggested, but one fact alone is well ascertained. Certain men of San Miniato, having been expelled as rebels to the Empire, had sought the Bishop of Florence[160] in his palace, and sworn not only to make common cause with the Pisans and Florentines, but to cede them the territory of San Miniato, should they succeed in retaking it, and even if the fortress remained in the hands of the Germans.[161] This is certainly true, for the document containing the agreement is still extant. It is no regular treaty, being unwitnessed by Consuls, and lacking the proper legal formulas. But the fact of its having been sworn to and signed in the bishop's palace; of some leading citizens, including one of the Uberti,[162] having been parties to it; and of the document being preserved in the Archives,[163] proves that the rulers of the two cities were not unaware of the agreement, but merely preferred to hide, or rather disguise the real importance of it. All this, joined to their reluctance and delay as to the exchange of prisoners, persuaded Christian that they were trying to trick and betray him by a fictitious peace. Accordingly, his patience being exhausted, he was led to commit an imprudent and ill-considered action, that destroyed all hope of the peace he was so anxious to conclude.