 Complete the following phrase:  Finally, in late 1229, Theodore began assembling his forces at Thessalonicaincluding a contingent of troops sent by Frederick IIfor the projected final attack on Constantinople. As his troops marched east in spring 1230, he unexpectedly turned his army north and followed the Evros valley into Bulgaria. Theodores motives for this sudden change are debated. Contemporary and later historians like Akropolites denounced this as a sign of Theodores duplicity and treachery. A more likely suggestion is that Theodore, hitherto undefeated in battle, desired to check Bulgarian power and avoid the possibility of the Bulgarians striking in his rear while he was engaged in besieging Constantinople. Although caught by surprise, John Asen II reacted rapidly. Allegedly affixing the text of the broken treaty to one of his spears as a flag, he rallied his troops and met Theodore in battle in April 1230. The ensuing 