Finally, in late 1229, Theodore began assembling his forces at Thessalonica—including a contingent of troops sent by Frederick II—for 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. Theodore's motives for this sudden change are debated. Contemporary and later historians like Akropolites denounced this as a sign of Theodore's 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 Battle of Klokotnitsa was a crushing victory for the Bulgarian ruler, who took Theodore and many of his officers captive.