As the fall of Thessalonica appeared imminent, Pope Honorius excommunicated Theodore, ordered an embargo on horses, troops, and supplies from the Adriatic ports, and sent letters to Constantinople urging assistance to Demetrius. Demetrius himself went to Italy to seek aid, being received by the Pope in Rome in March 1222 and by Emperor Frederick II () soon after. A crusade was proclaimed against Theodore, and troops began to gather in Italy. In the meantime, the Latin Empire, now under Robert of Courtenay (), was once more embroiled in warfare with Nicaea as it supported Laskaris' brothers in their challenge against the new Nicaean emperor, John III Doukas Vatatzes (). The first detachments of the assembling crusade, under Count Oberto II of Biandrate, arrived at Thessalonica in summer 1222 and joined the "de facto" regent, Guy Pallavicini. Theodore now hastened his move against Thessalonica. After preliminary operations in late 1222, in early 1223 he laid siege to the city. Honorius excommunicated him again, and redoubled his effort to promote the crusade, which had come to a standstill in the meantime. At the Pope's urging, Venice and Emperor Frederick promised aid. Robert of Courtenay also pledged to assist, while Honorius called upon the Latin princes of southern Greece to join in the effort. The crusade finally assembled at Brindisi in March 1224. In his instructions to his legate, however, Honorius did not rule out the possibility of Theodore coming to terms with the Crusaders. True to his word, in April 1224 Robert of Courtenay sent an army to besiege Serres. During the siege, the Latin generals learned of the disastrous defeat of the main Latin army at the hands of John III Doukas Vatatzes at the Battle of Poimanenon. They lifted the siege and hastened back to Constantinople, only to be intercepted by Theodore's men; most of the Latin army were killed or taken prisoner.